Sai Gu

Professor Sai Gu


Professor of Chemical Engineering
+44 (0)1483 68
18 BC 02

Academic and research departments

Department of Chemical and Process Engineering.

Biography

Research

Research interests

My publications

Publications

Laura Pastor Perez, Estelle Le Saché, C. Jones, Sai Gu, Harvey Arellano-Garcia, Tomas Ramirez Reina (2017)Synthetic natural gas production from CO 2 over Ni-x/CeO 2 -ZrO 2 (x = Fe, Co) catalysts: Influence of promoters and space velocity, In: Catalysis Today317pp. 108-113 Elsevier

Herein, the production of synthetic natural gas is proposed as an effective route for CO2 conversion. Typical catalysts for this reaction are based on Ni. In this study, we demonstrated that the addition of promoters such as iron and cobalt can greatly benefit the activity of standard Ni methanation catalysts. In particular cobalt seems to be a very efficient promoter. Our Co doped material is an outstanding catalysts for the CO2 methanation leading to high levels of CO2 conversion with selectivities close to 100%. Additionally, this catalyst is able to preserve excellent performance at relatively high space velocity which allows flexibility in the reactor design making easier the development of compact CO2 utilisation units. As an additional advantage, the Co-promoted catalysts is exceptionally stable conserving high levels of CO2 conversion under continuous operations in long terms runs.

Mobolaji Shemfe, Sai Gu, B Fidalgo (2017)Techno-economic analysis of biofuel production via bio-oil zeolite upgrading: An evaluation of two catalyst regeneration systems, In: Biomass and Bioenergy98pp. 182-193 Elsevier

Biofuels have been identified as a mid-term GHG emission abatement solution for decarbonising the transport sector. This study examines the techno-economic analysis of biofuel production via biomass fast pyrolysis and subsequent bio-oil upgrading via zeolite cracking. The aim of this study is to compare the techno-economic feasibility of two conceptual catalyst regeneration configurations for the zeolite cracking process: (i) a two-stage regenerator operating sequentially in partial and complete combustion modes (P-2RG) and (ii) a single stage regenerator operating in complete combustion mode coupled with a catalyst cooler (P-1RGC). The designs were implemented in Aspen Plus® based on a hypothetical 72 t/day pine wood fast pyrolysis and zeolite cracking plant and compared in terms of energy efficiency and profitability. The energy efficiencies of P-2RG and P-1RGC were estimated at 54% and 52%, respectively with corresponding minimum fuel selling prices (MFSPs) of £7.48/GGE and £7.20/GGE. Sensitivity analysis revealed that the MFSPs of both designs are mainly sensitive to variations in fuel yield, operating cost and income tax. Furthermore, uncertainty analysis indicated that the likely range of the MFSPs of P-1RGC (£5.81/GGE  £11.63/GGE) at 95% probability was more economically favourable compared with P-2RG, along with a penalty of 2% reduction in energy efficiency. The results provide evidence to support the economic viability of biofuel production via zeolite cracking of pyrolysis-derived bio-oil.

Ioannis Kalargaris, G Tian, Sai Gu (2017)The utilisation of oils produced from plastic waste at different pyrolysis temperatures in a DI diesel engine, In: Energy Conversion and Management131pp. 179-185 Elsevier

Chemical recycling is an attractive way to address the explosive growth of plastic waste and disposal problems. Pyrolysis is a chemical recycling process that can convert plastics into high quality oil, which can then be utilised in internal combustion engines for power and heat generation. The aim of the present work is to evaluate the potential of using oils that have been derived from the pyrolysis of plastics at di erent temperatures in diesel engines. The produced oils were analysed and found to have similar properties to diesel fuel. The plastic pyrolysis oils were then tested in a four-cylinder direct injection diesel engine, and their combustion, performance and emission characteristics analysed and compared to mineral diesel. The engine was found to perform better on the pyrolysis oils at higher loads. The pyrolysis temperature had a signi cant e ect, as the oil produced at a lower temperature presented higher brake thermal e ciency and shorter ignition delay period at all loads. This oil also produced lower NOX, UHC, CO and CO2 emissions than the oil produced at a higher temperature, although diesel emissions were lower.

S Das, A Bhattacharya, S Haldar, A Ganguly, S Gu, YP Ting, PK Chatterjee (2015)Optimization of enzymatic saccharification of water hyacinth biomass for bio-ethanol: Comparison between artificial neural network and response surface methodology, In: Sustainable Materials and Technologies3(April)pp. 17-28 Elsevier B.V.

Response surface methodology (RSM) is commonly used for optimising process parameters affecting enzymatic hydrolysis. However, artificial neural network–genetic algorithm hybrid model can also serve as an effective option, primarily for non-linear polynomial systems. The present study compares these approaches for enzymatic hydrolysis of water hyacinth biomass to maximise total reducing sugar (TRS) for bio-ethanol production. Maximum TRS (0.5672 g/g) was obtained using 9.92 (% w/w) substrate concentrations, 49.56 U/g cellulase concentrations, 280.33 U/g xylanase concentrations and 0.13 (% w/w) surfactant concentrations. The average % error for artificial neural networking (ANN) and RSM were 3.08 and 4.82 and the prediction percentage errors in optimum output are 0.95 and 1.41, respectively, which showed the supremacy of ANN in illustrating the non-linear behaviour of the system. Fermentation of the hydrolysate yielded a maximum ethanol concentration of 10.44 g/l using Pichia stipitis, followed by 8.24 and 6.76 g/l for Candida shehatae and Saccharomyces cerevisiae.

Q Eri, X Zhao, P Ranganathan, Sai Gu (2017)Numerical simulations on the effect of potassium on the biomass fast pyrolysis in fluidized bed reactor, In: Fuel197pp. 290-297 Elsevier

In this study, the effect of potassium on the cellulose fast pyrolysis in fluidized bed reactor has been studied using Computational Fluid Dynamics (CFD). A multiphase pyrolysis model of cellulose has been implemented by integrating the hydrodynamics of the fluidized bed with an adjusted cellulose pyrolysis mechanism that accounts the effect of potassium. The model has been validated with the reported experimental data. The simulation results show that potassium concentration and reactor temperature have a significant effect on the yields and components of cellulose pyrolysis products. The product yields fluctuate is caused by the unstable flow in the fluidized bed. The result shows that the increased potassium concentration in the cellulose causes a significant increase of the gas and char yields and reduction in the bio-oil. Also, the dramatic composition variations in bio-oil and gas were observed due to the inhibition of fragmentation, and the depolymerization reaction of activated cellulose, and the catalysis of the depolymerization reaction of cellulose. It is also found that the increase in reactor temperature greatly enhances the endothermic pyrolysis reaction, which leads to the significant changes in the yield and composition of cellulose pyrolysis products.

C Loha, S Gu, J De Wilde, P Mahanta, PK Chatterjee (2014)Advances in mathematical modeling of fluidized bed gasification, In: Renewable and Sustainable Energy Reviews40(Decemb)pp. 688-715 ScienceDirect

Gasification is the thermochemical conversion of solid fuel into the gas which contains mainly hydrogen, carbon monoxide, carbon dioxide, methane and nitrogen. In gasification, fluidized bed technology is widely used due to its various advantageous features which include high heat transfer, uniform and controllable temperature and favorable gas–solid contacting. Modeling and simulation of fluidized bed gasification is useful for optimizing the gasifier design and operation with minimal temporal and financial cost. The present work investigates the different modeling approaches applied to the fluidized bed gasification systems. These models are broadly classified as the equilibrium model and the rate based or kinetic model. On the other hand, depending on the description of the hydrodynamic of the bed, fluidized bed models may also be classified as the two-phase flow model, the Euler–Euler model and the Euler–Lagrange model. Mathematical formulation of each of the model mentioned above and their merits and demerits are discussed. Detail reviews of different model used by different researchers with major results obtained by them are presented while the special focus is given on Euler–Euler and Euler–Lagrange CFD models.

Wei Jin, L. Pastor- Pérez, J. Yu, J.A. Odriozola, S. Gu, T.R Reina (2019)Cost-effective routes for Catalytic Biomass Upgrading, In: Current Opinion in Green and Sustainable Chemistry

Catalytic hydrodeoxygenation (HDO) is a fundamental and promising route for bio-oil upgrading to produce petroleum-like hydrocarbon fuels or chemical building blocks. One of the main challenges of this technology is the demand of high-pressure H2, which poses high costs and safety concerns. Accordingly, developing cost-effective routes for biomass or bio-oil upgrading without the supply of commercial H2 is essential to implement the HDO at commercial scale. This paper critically reviewed the very recent studies relating to the novel strategies for upgrading the bio-feedstocks with ‘green’ H2 generated from renewable sources. More precisely, catalytic transfer hydrogenation/hydrogenolysis (CTH), combined reforming and HDO, combined metal hydrolysis and HDO, water-assisted in-situ HDO and non-thermal plasma (NTP) technology and self-supported hydrogenolysis (SSH) are reviewed herein. Current challenges and research trends of each strategy are also proposed aiming to motivate further improvement of these novel routes to become competitive alternatives to conventional HDO technology.

J Singh, S Gu (2010)Commercialization potential of microalgae for bio fuels production, In: Renewable and Sustainable Energy Reviews14(9, Dec)pp. 2596-2610 ScienceDirect

Microalgae feedstocks are gaining interest in the present day energy scenario due to their fast growth potential coupled with relatively high lipid, carbohydrate and nutrients contents. All of these properties render them an excellent source for biofuels such as biodiesel, bioethanol and biomethane; as well as a number of other valuable pharmaceutical and nutraceutical products. The present review is a critical appraisal of the commercialization potential of microalgae biofuels. The available literature on various aspects of microalgae, e.g. its cultivation, life cycle assessment, and conceptualization of an algal biorefinery, has been scanned and a critical analysis has been presented. A critical evaluation of the available information suggests that the economic viability of the process in terms of minimizing the operational and maintenance cost along with maximization of oil-rich microalgae production is the key factor, for successful commercialization of microalgae-based fuels.

Wei Jin, Laura Pastor-Pérez, Juan J. Villora-Pico, Mercedes M. Pastor-Blas, Antonio Sepúlveda-Escribano, Sai Gu, Nikolaos D. Charisiou, Kyriakos Papageridis, Maria A. Goula, Tomas R. Reina (2019)Catalytic Conversion of Palm Oil to Bio-Hydrogenated Diesel over Novel N-Doped Activated Carbon Supported Pt Nanoparticles, In: Energies13(1) MDPI

Bio-hydrogenated diesel (BHD), derived from vegetable oil via hydrotreating technology, is a promising alternative transportation fuel to replace nonsustainable petroleum diesel. In this work, a novel Pt-based catalyst supported on N-doped activated carbon prepared from polypyrrole as the nitrogen source (Pt/N-AC) was developed and applied in the palm oil deoxygenation process to produce BHD in a fixed bed reactor system. High conversion rates of triglycerides (conversion of TG > 90%) and high deoxygenation percentage (DeCOx% = 76% and HDO% = 7%) were obtained for the palm oil deoxygenation over Pt/N-AC catalyst at optimised reaction conditions: T = 300 ◦C, 30 bar of H2, and LHSV = 1.5 h−1 . In addition to the excellent performance, the Pt/N-AC catalyst is highly stable in the deoxygenation reaction, as confirmed by the XRD and TEM analyses of the spent sample. The incorporation of N atoms in the carbon structure alters the electronic density of the catalyst, favouring the interaction with electrophilic groups such as carbonyls, and thus boosting the DeCOx route over the HDO pathway. Overall, this work showcases a promising route to produce added value bio-fuels from bio-compounds using advanced N-doped catalysts.

Elham Ketabchi, Evgenia Mechleri, Sai Gu, Harvey Arellano-Garcia (2018)Modelling and Optimisation Approach of an Integrated Oil Refinery and a Petrochemical Plant, In: Computer Aided Chemical Engineering (Part of volume: 13th International Symposium on Process Systems Engineering (PSE 2018))44pp. 1081-1086 Elsevier

An optimised integration approach connecting a conventional oil refinery with an ethylene production plant is investigated. Using the intermediate materials produced as the connection between the two plants, the use of internally provided feedstocks and blending options removes, at least partially, the reliance on external sourcing. This is also beneficial in terms of increasing profit margins and quality for both production systems. Thus, a mathematical model has been developed and implemented in this work to model the oil refinery and the ethylene production plant while considering their integration as an MINLP problem with the aim of optimising the integrated plants. This work considers the optimisation of each plant individually and later the final integration by modelling the interconnection between the oil refinery and the ethylene production plant. Moreover, a case study using practical data was carried out to verify the feasibility of the integration for an industrial application.

S Das, A Bhattacharya, A Ganguly, S Gu, PK Chatterjee (2014)Kinetics and genetic algorithm study of acid catalysed hydrolysis of water hyacinth biomass, In: Journal of Renewable Sustainable Energy6(063115) AIP Publishing

Industrialization has elevated our energy demand during the last century by many a folds, to deal with the rapid social growth. Non-renewable energy sources, like petroleum being the main source of energy, have become scarce due to their overuse and limited available reserve. Water hyacinth can be considered as a good source of lignocellulosic biomass. Cellulose and hemi-cellulose derived from water hyacinth via effective bioprocess can be converted to bioethanol. Production of liquid fuel from biomass can only be made economically viable and sustainable only if the de-polymerization of the recalcitrant hemi-cellulosic fraction of the biomass can be optimally utilized. The study aims to obtain an in-depth mechanistic understanding of the catalytic reaction involved in dilute sulphuric acid pre-treatment of water hyacinth biomass. Acid catalysed hemi-cellulose hydrolysis reaction kinetics in water hyacinth was studied based on a bi-phasic model. Arrhenius equation was used to study the kinetic modelling in a greater depth. A distinct relationship of soaking time of the biomass in the acid before hydrolysis on the activation energy and frequency factor was observed. A maximum xylose yield of 76.96% was predicted by the genetic algorithm based model for the optimum operating conditions; operating temperature (135.8 °C), concentration of sulphuric acid (5.6%), treatment time (17.45 min), and soaking time (3.99 h).

SA Nabavi, GT Vladisavljević, S Gu, EE Ekanem (2015)Double emulsion production in glass capillary microfluidic device: Parametric investigation of droplet generation behaviour, In: Chemical Engineering Science130(7 July)pp. 183-196 Elsevier B.V.

A three-phase axisymmetric numerical model based on Volume of Fluid–Continuum Surface Force (VOF–CSF) model was developed to perform parametric analysis of compound droplet production in three-phase glass capillary devices that combine co-flow and countercurrent flow focusing. The model predicted successfully generation of core–shell and multi-cored double emulsion droplets in dripping and jetting (narrowing and widening) regime and was used to investigate the effects of phase flow rates, fluid properties, and geometry on the size, morphology, and production rate of droplets. As the outer fluid flow rate increased, the size of compound droplets was reduced until a dripping-to-jetting transition occurred. By increasing the middle fluid flow rate, the size of compound droplets increased, which led to a widening jetting regime. The jetting was supressed by increasing the orifice size in the collection capillary or increasing the interfacial tension at the outer interface up to 0.06 N/m. The experimental and simulation results can be used to encapsulate CO2 solvents within gas-permeable microcapsules.

M Mahrukh, A Kumar, S Gu, S Kamnis (2016)Computational Development of a Novel Aerosol Synthesis Technique for Production of Dense and Nanostructured Zirconia Coating, In: Industrial and Engineering Chemistry Research55(28)pp. 7679-7695

The feasibility of a new processing method solution precursor high-velocity oxygen fuel spray (SP-HVOFS) is presented for the production of dense ZrO2-based nanostructured coatings, in which organometallic chemical precursor droplets are injected into the HVOF spray system. With the help of developed computational fluid dynamics (CFD) solver (Fluent), the evolution of particle volume, area, and number concentration is simulated considering nucleation, coagulation, and sintering. The aerosol model is validated with the experimental data available in the literature. When the oxygen-fuel gas flow rate (GFR) is increased, the (i) velocity and (ii) enthalpy of the HVOF flame is increased. The former reduces the particle residence time in the HVOF flame while the latter favors the sintering. Overall the results show that, by controlling the GFR, single scale nanometre particles (∼1–5 nm) can be fabricated without any agglomeration.

N Kishore, S Gu (2010)Wall Effects on Flow and Drag Phenomena of Spheroid Particles at Moderate Reynolds Numbers, In: Industrial and Engineering Chemistry Research49(19 (20)pp. 9486-9495 ACS Publications

Two dimensional steady Newtonian flow past oblate and prolate spheroid particles confined in cylindrical tubes of different diameters has been numerically investigated. The flow and drag phenomena of confined spheroid particles are governed by the equations of continuity and conservation of momentum. These equations along with appropriate boundary conditions have been solved using commercial software based on computational fluid dynamics. Extensive new results were obtained on individual and total drag coefficients of spheroid particles, along with streamline contours, distributions of pressure coefficients, and vorticity magnitudes on the surface of spheroid particles as functions of the Reynolds number (Re), the aspect ratio (e), and the wall factor (λ) over the following range of conditions: 1 ≤ Re ≤ 200, 0.25 ≤ e ≤ 2.5, and 2 ≤ λ ≤ 30. For all values of the aspect ratio, as values of the Reynolds numbers and/or the wall factor increase, the length of recirculation wake increases. For fixed values of the aspect ratio and the Reynolds number, the increase in the value of the wall factor decrease both individual and the total drag coefficients. On the whole, regardless of the value of the aspect ratio, the wall effect was found to gradually diminish with the increasing Reynolds number and/or the wall factor. Finally, on the basis of the present numerical results a simple correlation has been proposed for the total drag coefficient of confined spheroid particles which can be used in new applications.

Mobolaji Shemfe, C Whittaker, Sai Gu, B Fidalgo (2016)Comparative evaluation of GHG emissions from the use of Miscanthus for bio-hydrocarbon production via fast pyrolysis and bio-oil upgrading, In: Applied Energy176pp. 22-33 Elsevier

This study examines the GHG emissions associated with producing bio-hydrocarbons via fast pyrolysis of Miscanthus. The feedstock is then upgraded to bio-oil products via hydroprocessing and zeolite cracking. Inventory data for this study were obtained from current commercial cultivation practices of Miscanthus in the UK and state-of-the-art process models developed in Aspen Plus®. The system boundary considered spans from the cultivation of Miscanthus to conversion of the pyrolysis-derived bio-oil into bio-hydrocarbons up to the refinery gate. The Miscanthus cultivation subsystem considers three scenarios for soil organic carbon (SOC) sequestration rates. These were assumed as follows: (i) excluding (SOC), (ii) low SOC and (iii) high (SOC) for best and worst cases. Overall, Miscanthus cultivation contributed moderate to negative values to GHG emissions, from analysis of excluding SOC to high SOC scenarios. Furthermore, the rate of SOC in the Miscanthus cultivation subsystem has significant effects on total GHG emissions. Where SOC is excluded, the fast pyrolysis subsystem shows the highest positive contribution to GHG emissions, while the credit for exported electricity was the main ‘negative’ GHG emission contributor for both upgrading pathways. Comparison between the bio-hydrocarbons produced from the two upgrading routes and fossil fuels indicates GHG emission savings between 68 and 87%. Sensitivity analysis reveals that bio-hydrocarbon yield and nitrogen gas feed to the fast pyrolysis reactor are the main parameters that influence the total GHG emissions for both pathways.

A Kumar, S Gu (2012)Modelling impingement of hollow metal droplets onto a flat surface, In: International Journal of Heat and Fluid Flow37(Octobe)pp. 189-195 ScienceDirect

Despite many theoretical and experimental works dealing with the impact of dense melt droplets on the substrate during the process of thermal spray coating, the dynamics of the impingement of hollow melt droplet and the subsequent splat formation are not well addressed. In this paper a model study for the dynamic impingement of hollow droplet is presented. The hollow droplet is modelled such that it consists of a liquid shell enclosing a gas cavity. The impingement model considers the transient flow dynamics during impact, spreading and solidification of the droplet using the volume of fluid surface tracking method (VOF) coupled with a solidification model within a one-domain continuum formulation. The results for spreading, solidification and formation of splats clearly show that the impingement process of hollow droplet is distinctly different from the dense droplet. Study with different droplet void fractions and void distribution indicates that void fraction and void distribution have a significant influence on the flow dynamics during impact and on the final splat shape. The results are likely to provide insights for the less-explored behaviour of hollow melt droplets in thermal spray coating processes.

M Mahrukh, A Kumar, S Gu, S Kamnis, E Gozali (2016)Modeling the Effects of Concentration of Solid Nanoparticles in Liquid Feedstock Injection on High-Velocity Suspension Flame Spray Process, In: Industrial and Engineering Chemistry Research55(9)pp. 2556-2573 AMER CHEMICAL SOC

This paper presents the effects of the concentration of solid nanoparticles in the liquid feedstock injection on the high-velocity suspension flame spray (HVSFS) process. Four different concentrations of solid nanoparticles in suspension droplets with various droplet diameters are used to study gas dynamics, vaporization rate, and secondary breakup. Two types of injections, viz. surface and group, are used. The group-type injection increases the efficiency of droplet disintegration and the evaporation process and reduces the gas cooling. The initiation of the fragmentation process is difficult for small droplets carrying a high concentration of nanoparticles. Also, smaller droplets undergo rapid vaporization, leaving clogs of nanoparticles in the middle of the barrel. For larger droplets, severe fragmentation occurs inside the combustion chamber. For a higher concentration of nanoparticles, droplets exit the gun without complete evaporation. The results suggest that, in coating applications involving a higher concentration of nanoparticles, smaller droplet sizes are preferred.

Polymer electrolyte membrane (PEM) fuel cells have higher efficiency and energy density and are capable of rapidly adjusting to power demands. Effective water management is one of the key issues for increasing the efficiency of PEMFC. In the current study, a three-dimensional (3D) lattice Boltzmann model is developed to simulate the water transport and oxygen diffusion in the gas diffusion layer (GDL) of PEM fuel cells with electrochemical reaction on the catalyst layer taken into account. In this paper, we demonstrate that this model is able to predict the liquid and gas flow fields within the 3D GDL structure and how they change with time. With the two-phase flow and electrochemical reaction coupled in the model, concentration of oxygen through the GDL and current density distribution can also be predicted. The model is then used to investigate the effect of microporous layer on the cell performance in 2D to reduce the computational cost. The results clearly show that the liquid water content can be reduced with the existence of microporous layer and thus the current density can be increased.

N Kishore, S Gu (2011)Effect of Blockage on Heat Transfer Phenomena of Spheroid Particles at Moderate Reynolds and Prandtl Numbers, In: Chemical Engineering Technology34(9, Sep)pp. 1551-1558

Effects of the confining wall or blockage on the heat transfer phenomena of spheroid particles were numerically investigated. The heated spheroid particles were maintained at constant temperature and allowed to sediment in cylindrical tubes filled with Newtonian liquids. In this flow configuration, the heat transfer took place from the heated spheroid particles to the surrounding Newtonian liquid. The governing conservation equations of the mass, momentum, and energy together with appropriate boundary conditions were numerically solved using commercial software based on computational fluid dynamics. A simple correlation for the average Nusselt number of the confined spheroid particles was developed which can be applied in new applications.

N Kishore, S Gu (2011)Momentum and heat transfer phenomena of spheroid particles at moderate Reynolds and Prandtl numbers, In: International Journal of Heat and Mass Transfer54(11-12,)pp. 2595-2601 Elsevier B.V.

The momentum and heat transfer phenomena of spheroid particles in an unbounded Newtonian fluid have been numerically investigated by solving governing conservation equations of the mass, the momentum and the energy. The numerical solution methodology has been benchmarked by performing comparisons between present results with those reported in the literature. Further, extensive new results have been obtained to elucidate effects of pertinent dimensionless parameters such as the Reynolds number (Re), the Prandtl number (Pr) and the aspect ratio (e) on the flow and heat transfer behaviour of spheroid particles in the range of parameters: 1 ⩽ Re ⩽ 200; 1 ⩽ Pr ⩽ 1000 and 0.25 ⩽ e ⩽ 2.5. Regardless of the value of the Reynolds number, the total and individual drag coefficients of oblate spheroids (e < 1) are smaller than those of spheres (e = 1) and opposite trend has been observed for prolate spheroids (e > 1). Irrespective of values of Reynolds and Peclet numbers, the average Nusselt number is large for prolate particles as compared to spheres and opposite trend has been observed for the case of oblate particles. Major contribution of this work is the development of simple correlations for the total drag coefficient and the average Nusselt number of unconfined isolated spheroid particles based on present numerical results which can be used in new applications.

Z Zhu, S Kamnis, S Gu (2015)Numerical study of molten and semi-molten ceramic impingement by using coupled Eulerian and Lagrangian method, In: Acta Materialia90(15 May)pp. 77-87 Elsevier B.V.

Large temperature gradients are present within ceramic powder particles during plasma spray deposition due to their low thermal conductivity. The particles often impinge at the substrate in a semi-molten form which in turn substantially affects the final characteristics of the coating being formed. This study is dedicated to a novel modeling approach of a coupled Eulerian and Lagrangian (CEL) method for both fully molten and semi-molten droplet impingement processes. The simulation provides an insight to the deformation mechanism of the solid core YSZ and illustrates the freezing-induced break-up and spreading at the splat periphery. A 30 μm fully molten YSZ particle and an 80 μm semi-molten YSZ particle with different core sizes and initial velocity ranging from 100 to 240 m/s were examined. The flattened degree for both cases were obtained and compared with experimental and analytical data.

W. Jin, L. Pastor-Perez, Juan J. Villora-Picó, A. Sepúlveda-Escribano, S. Gu, T. R. Reina (2019)Investigating New Routes for Biomass Upgrading: “H2-Free” Hydrodeoxygenation Using Ni-Based Catalysts, In: ACS Sustainable Chemistry & Engineering7(19)pp. 16041-16049 American Chemical Society

This work showcases an innovative route for biocompound upgrading via hydrodeoxygenation (HDO) reactions, eliminating the need for external high-pressure hydrogen supply. We propose the use of water as reaction media and the utilization of multifunctional catalysts that are able to conduct multiple steps such as water activation and HDO. In this study, we validate our hypothesis in a high-pressure batch reactor process using guaiacol as a model compound and multicomponent Ni-based catalysts. In particular, a comparison between ceria-supported and carbon/ceria-supported samples is established, the carbon-based materials being the suitable choice for this reaction. The physicochemical study by X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction, and temperature-programmed reduction reveals the greater dispersion of Ni clusters and the strong metal-support interaction in the carbon/ceria-based samples accounting for the enhanced performance. In addition, the characterization of the spent samples points out the resistance of our catalysts toward sintering and coking. Overall, the novel catalytic approach proposed in this paper opens new research possibilities to achieve low-cost bio-oil upgrading processes.

When a complex geometry is rotated in front of the thermal spray gun, the following kinematic parameters vary in a coupled fashion dictated by the geometry: Stand-off distance, spray angle and gun traverse speed. These fluctuations affect the conditions of particle impact with major implications on the coating’s properties. This work aims to probe into the interplay and isolated effect of these parameters on vital coating characteristics in applications requiring variable stand-off distance and spray angles. WC-17Co powders are sprayed via HVOF on steel substrates in a set of experiments that simulates the spray process of a non-circular cross section, while it allows for individual control of the kinematic parameters. Comprehensive investigation of their influence is made on deposition rate, residual stresses, porosity and microhardness of the final coating. It was determined that oblique spray angles and long stand-off distances compromise the coating properties but in some cases, the interplay of the kinematic parameters produced non-linear behaviours. Microhardness is related negatively with oblique spray angles at short distances while a positive correlation emerges as the stand-off distance is increased. Porosity and residual stresses are sensitive to the spray angle only in relatively short stand-off distances.

Siddharth Gadkari, Sai Gu, Jhuma Sadhukhan (2018)Towards automated design of bioelectrochemical systems: A comprehensive review of mathematical models, In: Chemical Engineering Journal343pp. 303-316 Elsevier

This review presents the developments in the mathematical models for various bioelectrochemical systems. A number of modeling approaches starting with the simple description of biological and electrochemical processes in terms of ordinary differential equations to very detailed 2D and 3D models that study the spatial distribution of substrates and biomass, have been developed to study BES performance. Additionally, mathematical models focused on studying a particular process such as ion diffusion through membrane and new modeling approaches such as artifcial intelligence methods, cellular network models, etc., have also been described. While most mathematical models are still focused on performance studies and optimization of microbial fuel cells, new models to study other BESs such as microbial electrolysis cell, microbial electrosynthesis and microbial desalination cell have also been reported and discussed in this review.

NH Dong, LM Armstrong, S Gu, KH Luo (2012)Effect of tube shape on the hydrodynamics and tube-to-bed heat transfer in fluidized beds, In: Applied Thermal Engineering60(1-2, 2)pp. 472-479 ScienceDirect

In the present work, the multiphase flow dynamics in fluidized beds is modelled using the Two-Fluid Model (TFM) where the characteristics of a granular solid phase are described by the Kinetic Theory of Granular Flow (KTGF). A drag function and heat transfer coefficients are used to describe the interaction and heat exchange between different phases, respectively. The effective thermal conductivity is defined as a function of phase volume fraction and thermal properties and is used to calculate the heat transfer coefficient from immersed tube to fluidized beds. The effects of different tube shapes on the flow characteristics and local heat transfer coefficients are investigated and the time-averaged heat-transfer coefficient is compared with the experimental data in the literature. The simulated results show that the heat transfer processes are significantly influenced by the reintroduction of solid particles around the immersed surfaces and the heat transfer coefficients vary sensitively with the distribution of the solid phase. The simulated heat transfer coefficients are in the same order as the experimental data which indicates that it can be quantitatively employed to aid the configuration of heating tubes during industrial design of the fluidized bed reactors.

Ioannis Kalargaris, Guohong Tian, Sai Gu (2017)Experimental evaluation of a diesel engine fuelled by pyrolysis oils produced from low-density polyethylene and ethylene-vinyl acetate plastics, In: Fuel Processing Technology161pp. 125-131 Elsevier

Depletion of oil resources and increase in energy demand have driven the researchers to seek ways to convert the waste products into high quality oils that could replace fossil fuels. Plastic waste is in abundance and can be converted into high quality oil through the pyrolysis process. In this study, pyrolysis oils were produced from polyethylene (LDPE700), the most common used plastic, and ethylene-vinyl acetate (EVA900) at pyrolysis temperatures of 700oC and 900oC respectively. The oils were then tested in a four cylinder diesel engine, and the performance, combustion and emission characteristics were analysed in comparison with mineral diesel. It was found that the engine could operate on both oils without the addition of diesel. LDPE700 exhibited almost identical combustion characteristics and brake thermal e ciency to that of diesel operation, with lower NOX, CO and CO2 emissions but higher unburned hydrocarbons (UHC). On the contrary, EVA900 presented longer ignition delay period, lower e ciency (1.5–2%), higher NOX and UHC emissions and lower CO and CO2 in comparison to diesel. The addition of diesel to the EVA900 did not significantly improve the overall engine performance.

Laura Pastor-Perez, Sai Gu, Antonio Sepúlveda-Escribano, Tomas R. Reina (2019)Bimetallic Cu-Ni catalysts for the WGS reaction- Cooperative or uncooperative effect?, In: International Journal of Hydrogen Energy44(8)pp. pp 4011-4019 Elsevier

In this work, bimetallic Cu–Ni catalysts have been studied in the water-gas shift (WGS) reaction, and they have shown different levels of synergy and anti-synergy in terms of catalytic activity and selectivity to the desired products. Cu–Ni interactions alter the physicochemical properties of the prepared materials (i.e. surface chemistry, redox behaviour, etc.) and as a result, the catalytic trends are influenced by the catalysts' composition. Our study reveals that Cu enhances Ni selectivity to CO2 and H2 by preventing CO/CO2 methanation, while Ni does not help to improve Cu catalytic performance by any means. Indeed, the monometallic Cu formulation has shown the best results in this study, yielding high levels of reactants conversion and excellent long-term stability. Interestingly, for medium-high temperatures, the bimetallic 1Cu–1Ni outperforms the stability levels reached with the monometallic formulation and becomes an interesting choice even when start-up/shutdowns operations are considered during the catalytic experiments.

Polypropylene is the most common type of plastic found in municipal solid waste. The production of polypropylene is expected to increase due to the widespread utilization in daily life, resulting in even higher amounts of polypropylene waste. Sending this plastic to landfill not only exacerbates environmental problems, but also results in energy loss due to the elevated energy content of polypropylene. Pyrolysis is a process that can effectively convert polypropylene waste into fuel, which can then be used to generate power and heat. In the present study, the ect of the pyrolysis temperature on the pyrolysis of polypropylene was investigated, and the oils produced at 700oC (PP700) and 900oC (PP900) were used to fuel a four cylinder diesel engine. The engine's combustion, performance and emission characteristics were analysed and compared to diesel operation. The results showed that both PP700 and PP900 enabled stable engine operation, with PP900 performing slightly better in terms of efficiency and emissions. However, PP700 and PP900 were found to have longer ignition delay periods, longer combustion periods, lower brake thermal efficiencies, higher NOX, UHC and CO emissions, and lower CO2 emissions in comparison to diesel operation. Nonetheless, the addition of a small quantity of diesel improved the overall performance of the oil blends, resulting in comparable results to diesel in the case of PP900.

Monica Garcia Ortega, H Knuutila, Sai Gu (2017)ASPEN PLUS simulation model for CO2 removal with MEA: Validation of desorption model with experimental data, In: Journal of Environmental Chemical Engineering5pp. 4693-4701 Elsevier

The chemical absorption process has been extensively studied as one of the main carbon capture and separation technologies. This process comprises two stages: The absorption of CO2 into the solvent and the desorption, to regenerate the solvent and produce the high concentrated CO2 gas. Validated simulation models are essential for the scale-up of the chemical absorption process and they are typically validated using only data from one pilot plant. In this work, a simulation model of the desorption column built in ASPEN PLUS v8.6 was validated using four experimental pilot campaigns using 30 wt% MEA. The desorbers in the different campaigns varied in the diameters, structured packing heights and packing types. A good agreement is observed between experimental data and the simulation results of the chemical absorption process presented here. The model shows an AARD (average absolute relative deviation) of 9.2% for the CO2 stripped (kg/h) for the tested 78 experimental runs. The simulated temperatures of the liquid flux leaving the reboiler show a deviation of 3.3% compared with the experimental data. The deviations on the estimation of the CO2 stripped show some dependency on the CO2 loading in the rich amine flux entering the desorber. However, the deviations are independent on the temperature of the rich amine

The present work concerns with CFD modelling of biomass fast pyrolysis in a fluidised bed reactor. Initially, a study was conducted to understand the hydrodynamics of the fluidised bed reactor by investigating the particle density and size, and gas velocity effect. With the basic understanding of hydrodynamics, the study was further extended to investigate the different kinetic schemes for biomass fast pyrolysis process. The Eulerian-Eulerian approach was used to model the complex multiphase flows in the reactor. The yield of the products from the simulation was compared with the experimental data. A good comparison was obtained between the literature results and CFD simulation. It is also found that CFD prediction with the advanced kinetic scheme is better when compared to other schemes. With the confidence obtained from the CFD models, a parametric study was carried out to study the effect of biomass particle type and size and temperature on the yield of the products.

Wei Jin, Jose Luis Santos, Laura Pastor-Perez, Sai Gu, Miguel Angel Centeno, Tomas Ramirez Reina (2019)Noble metal supported on activated carbon for "hydrogen free" HDO reactions: exploring economically advantageous routes for biomass valorisation, In: ChemCatChem Wiley

An innovative route for bio‐compounds upgrading via “hydrogen‐free” hydrodeoxygenation (HDO) is proposed and evaluated using guaiacol as a model compound in a high‐pressure batch reactor. Experimental results showed that noble metal supported on activated carbon catalysts are able to conduct tandem multiple steps including water splitting and subsequent HDO. The activity of Ru/C catalyst is superior to other studied catalysts (i.e. Au/C, Pd/C and Rh/C) in our water‐only HDO reaction system. The greater dispersion and smaller metal particle size confirmed by the TEM micrographs accounts for the better performance of Ru/C. This material also presents excellent levels of stability as demonstrated in multiple reciclabylity runs. Overall, the proposed novel approach confirmed the viability of oxygenated bio‐compounds upgrading in a water‐only reaction system suppressing the need of external H2 supply and can be rendered as a fundamental finding for the economical biomass valorisation to produce added value bio‐fuels.

Daniel Sebastia Saez, Sai Gu, Panneerselvam Ranganathan, Konstantinos Papadikis (2014)Micro-scale CFD modeling of reactive mass transfer in falling liquid films within structured packing materials, In: International Journal of Greenhouse Gas Control33(Februa)pp. 40-50 Elsevier B.V.

Post-combustion carbon capture in structured packing columns is considered as a promising technology to reduce greenhouse gas (GHG) emissions because of its maturity and the possibility of being retrofitted to existing power plants. CFD plays an important role in the optimization of this technology. However, due to the current computational capacity limitations, the simulations need to be divided into three scales (i.e. micro-, meso- and macro-scale) depending on the flow characteristics to be analyzed. This study presents a 3D micro-scale approach to describe the hydrodynamics and reactive mass transfer of the CO2-MEA chemical system within structured packing materials. Higbie's penetration theory is used to describe the mass transfer characteristics whereas enhancement factors are implemented to represent the gain in the absorption rate attributable to the chemical reaction. The results show a detrimental effect of the liquid load on the absorption rate via a decrease in the enhancement factor. The evolution of the wetted area for MEA solutions is compared to the case of pure water highlighting the differences in the transient behavior. The CO2 concentration profiles are examined showing the capability of the model to reproduce the depletion of the solute within the bulk liquid ascribed to the high value of the Hatta number. Also, several approaches on the reaction mechanism such as reversibility and instantaneous behavior are assessed. The results from micro-scale are to be used in meso-scale analysis in future studies to optimize the reactive absorption characteristics of structured packing materials.

Utsab Guharoy, Tomas Ramirez Reina, Sai Gu, Qiong Cai (2019)Mechanistic Insights into Selective CO2 Conversion via RWGS on Transition Metal Phosphides: A DFT Study, In: Journal of Physical Chemistry C123(37)pp. 22918-22931 American Chemical Society

Selective conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is an attractive CO2 conversion process, which may be integrated with many industrial catalytic processes such as Fischer−Tropsch synthesis to generate added value products. The development of active and cost friendly catalysts is of paramount importance. Among the available catalyst materials, transition metal phosphides (TMPs) such as MoP and Ni2P have remained unexplored in the context of the RWGS reaction. In the present work, we have employed density functional theory (DFT) to first investigate the stability and geometries of selected RWGS intermediates on the MoP (0001) surface, in comparison to the Ni2P (0001) surface. Higher adsorption energies and Bader charges are observed on MoP (0001), indicating better stability of intermediates on the MoP (0001) surface. Furthermore, mechanistic investigation using potential energy surface (PES) profiles showcased that both MoP and Ni2P were active toward RWGS reaction with the direct path (CO2* → CO* + O*) favorable on MoP (0001), whereas the COOH-mediated path (CO2* + H* → COOH*) favors Ni2P (0001) for product (CO and H2O) gas generation. Additionally, PES profiles of initial steps to CO activation revealed that direct CO decomposition to C* and O* is favored only on MoP (0001), while H-assisted CO activation is more favorable on Ni2P (0001) but could also occur on MoP (0001). Furthermore, our DFT calculations also ascertained the possibility of methane formation on Ni2P (0001) during the RWGS process, while MoP (0001) remained more selective toward CO generation.

N Zeoli, S Gu (2006)Numerical modelling of droplet break-up for gas atomisation, In: Computational Materials Science38(2, Dec)pp. 282-292 ScienceDirect

High-pressure gas atomisation (HPGA) technology has been widely employed as an effective method to produce fine spherical metal powders. The physics of gas atomisation is dominated by rapid momentum and heat transfer between the gas and melt phases, and further complicated by break-up and solidification. A numerical model is developed to simulate the critical droplet break-up during the atomisation. By integration of the droplet break-up model with the flow field generated high-pressure gas nozzle, this numerical model is able to provide quantitative assessment for atomisation process. To verify the model performance, the melt stream is initialized to large droplets varying from 1 to 5 mm diameters and injected into the gas flow field for further fragmentation and the break-up dynamics are described in details according to the droplet input parameters.

Juan Daniel Sebastia Saez, Sai Gu, L. Konozsy, J.-U. Repke, Harvey Arellano-Garcia (2017)On the effect of the Froude number on the interface area of gravity-driven liquid rivulets, In: Chemical Engineering Research and Design130pp. 208-218 Elsevier

The morphology of gravity-driven rivulets affects the mass transfer performance in gas separation processes, hence, the need for an improved knowledge on the hydrodynamics of this ow. It is well established that the interface area of the rivulets is determined by the balance between inertia and surface tension, i.e. the Weber number, which in light of the results presented here, are not the only parameters involved, but also the inclination of the plate has an effect on the balance of forces which determines the amount of gas-liquid interface area. The analysis of the interface area in rivulet ow demands, therefore, a more complete physical explanation for packing design purposes. In this work, we analyse the combined effect of both the inertia and the inclination of the plate in the interface area of liquid rivulets using CFD and the Volume-of-Fluid interface tracking method. As a result, we propose the use of the Froude number to provide a more complete physical explanation on the interface area formation of gravity-driven liquid rivulets.

S Kamnis, S Gu (2005)Numerical modelling of propane combustion in a high velocity oxygen–fuel thermal spray gun, In: Chemical Engineering and Processing: Process Intensification45(4, Apr)pp. 246-253 ScienceDirect

High velocity oxygen–fuel (HVOF) thermal spraying is a relatively new technology compared to other protective coating methods. Powders sprayed by liquid fuel HVOF guns are able to achieve high impact velocities without overheating, which results in superior coatings. A computational fluid dynamic (CFD) model is developed to investigate propane combustion in the process of HVOF thermal spraying. The numerical methods are described for correct representation of various thermal–physical phenomena such as flame propagation, turbulent mixing and flow acceleration. The principal advantages and shortcomings of various models are discussed.

N Kamnis, S Gu (2006)3-D modelling of kerosene-fuelled HVOF thermal spray gun, In: Chemical Engineering Science61(16, Au)pp. 5427-5439 ScienceDirect

Liquid-fuelled high-velocity oxy-fuel (HVOF) thermal spraying systems are capable of generating more momentum output to powder particles in comparison with gas-fuelled systems. The use of low-cost fuel such as kerosene makes this technology particular attractive. High-quality coating requires thermal spraying systems delivering consistent performance as a result of the combustion during HVOF spraying. The combustion of kerosene is very complicated due to the variation of fuel composition and subsequently makes it extremely challenging for process control. This paper describes a 3-D simulation using mathematical models available in a commercial finite volume CFD code. The combustion and discrete particle models within the numerical code are applied to solve the combustion of kerosene and couple the motion of fuel droplets with the gas flow dynamics in a Lagrangian fashion. The effects of liquid fuel droplets on the thermodynamics of the combusting gas flow are examined thoroughly.

Ioannis Kalargaris, Guohong Tian, Sai Gu (2016)Combustion, performance and emission analysis of a DI diesel engine using plastic pyrolysis oil, In: Fuel Processing Technology157pp. 108-115 Elsevier

Plastic waste is an ideal source of energy due to its high heating value and abundance. It can be converted into oil through the pyrolysis process and utilised in internal combustion engines to produce power and heat. In the present work, plastic pyrolysis oil is manufactured via a fast pyrolysis process using a feedstock consisting of different types of plastic. The oil was analysed and it was found that its properties are similar to diesel fuel. The plastic pyrolysis oil was tested on a four-cylinder direct injection diesel engine running at various blends of plastic pyrolysis oil and diesel fuel from 0% to 100% at different engine loads from 25% to 100%. The engine combustion characteristics, performance and exhaust emissions were analysed and compared with diesel fuel operation. The results showed that the engine is able to run on plastic pyrolysis oil at high loads presenting similar performance to diesel while at lower loads the longer ignition delay period causes stability issues. The brake thermal efficiency for plastic pyrolysis oil at full load was slightly lower than diesel, but NOX emissions were considerably higher. The results suggested that the plastic pyrolysis oil is a promising alternative fuel for certain engine application at certain operation conditions.

N Zeoli, S Gu, S Kamnis (2007)Numerical simulation of in-flight particle oxidation during thermal spraying, In: Computers & Chemical Engineering32(7, 24)pp. 1661-1668 ScienceDirect

Metal powders are widely used for thermal spray coating to improve wear, corrosion and temperature resistance of products. The high thermal profiles endured for sprayed particles give rise to oxidation on the surface of metal powders. Metallic oxides are brittle and undermine the performance of coated products. To understand the growth of oxide layers on in-flight metal powders, an oxidation model is implemented into the Lagrangian formula of particle tracking. The numerical simulation is achieved for a 3D combusting gas flow generated by a high velocity oxygen fuel (HVOF) thermal spray gun. The results are able to demonstrate the correlation between in-flight particle oxidation and operation parameters.

N Zeoli, S Gu, S Kamnis (2008)Numerical modelling of metal droplet cooling and solidification, In: International Journal of Heat and Mass Transfer51(15-16,)pp. 4121-4131 ScienceDirect

In an atomisation process for power production, metal droplets go through undercooling, recalescence, peritectic and segregated solidification before fully solidified. The cooling process is further complicated by droplet break-up during the atomisation. This paper describes a numerical model which combines both cooling and break-up in a single computation. The dynamic history of droplets is solved as discrete phase in an Eulerian gas flow. The coupling between droplet and gas flows are two-way, in which the heat and momentum exchanges affecting the gas flow are treated as source/sink terms in the fluid equations. The droplet model is employed to a gas atomisation process for metal powder production and good agreement is achieved with the results in open literature. The model results further confirm that thermal history of particles is strongly dependent on initial droplet size. Large droplets will not go through undercooling while small droplets have identifiable stages of undercooling, unclearation and recalescence. The predictions demonstrate that droplets have very similar profiles during gas atomization and the major factor influencing the atomization and solidification process of droplets are in-flight distance.

N Zeoli, S Gu (2007)Computational simulation of metal droplet break-up, cooling and solidification during gas atomisation, In: Computational Materials Science43(2, Aug)pp. 268-278 ScienceDirect

In an atomisation process for powder production, metal droplets go through undercooling, recalescence, peritectic and segregated solidification before fully solidified. The cooling process is further complicated by droplet break-up during the atomisation. This paper describes a numerical model which combines both cooling and break-up in a single computation. The dynamic history of droplets is solved as discrete phase in an Eulerian gas flow. The coupling between droplet and gas flows are two-way, in which the heat and momentum exchanges affecting the gas flow are treated as source/sink terms in the fluid equations. The droplet models were employed a gas atomisation process for metal powder production and good agreement is achieved with the results in open literature. The model results further confirm that thermal history of particles is strongly dependent on initial droplet size. Large droplets will not go through undercooling while small droplets have identifiable stages of undercooling, unclearation and recalescence. The predictions demonstrate that droplets have very similar profiles during gas atomisation and the major factor influencing the atomisation and solidification process of droplets are in-flight distance.

S Gu, DG McCartney, CN Eastwick, KA Simmons (2003)Numerical Modelling of In-Flight Characteristics of Inconel 625 Particles during High Velocity Oxy-Fuel Thermal Spraying, In: Journal of Thermal Spray Technology13(2)pp. 200-213 Springer-Verlag

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 µm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.

N Zeoli, S Gu (2007)Computational validation of an isentropic plug nozzle design for gas atomisation, In: Computational Materials Science42(2, Apr)pp. 245-258 ScienceDirect

During high pressure gas atomisation (HPGA), the molten metal stream is disintegrated to produce spherical powders when energy is transferred from the gas to the melt. Conventional annular-slit nozzle (ASN) in close-coupled atomisation generates an under-expanded gas jet with characteristic shock waves which consume a great deal of energy through expansion. An isentropic plug nozzle (IPN) is developed in this paper to reduce the shocks and maximize kinetic energy being transferred from the gas to instablize the melt stream. The performance of the IPN is examined using a numerical model which includes gas flow dynamics, droplet break-up mechanism and particle tracking. The numerical results demonstrate a good improvement of gas dynamics and powder yield from the IPN design in comparison with the ASN, in particular when hot gas is employed.

In this work, a one-dimensional numerical uid model is developed for co-axial dielec- tric barrier discharge (DBD) in pure helium and a parametric study is performed to systematically study the in uence of relative permittivity of the dielectric barrier and the applied voltage amplitude and frequency on the discharge performance. Discharge current, gap voltage and spatially averaged electron density pro les are presented as a function of relative permittivity and voltage parameters. For the geometry un- der consideration, both the applied voltage parameters are shown to increase the maximum amplitude of the discharge current peak up to a certain threshold value, above which it stabilized or decreased slowly. The spatially averaged electron density pro les follow a similar trend as the discharge current. Relative permittivity of the dielectric barrier is predicted to have a positive in uence on the discharge current. At lower frequency it is also shown to lead a transition from Townsend to glow dis- charge mode. Spatially and time averaged power density is also calculated and is shown to increase with increasing relative permittivity, applied voltage amplitude and frequency.

K Kamnis, S Gu (2005)Numerical modelling of droplet impingement, In: Journal of Physics D: Applied Physics38(19 (20)pp. 3664-3673 IOP Publishing Ltd

Powder particles are projected from a thermal spray gun towards substrates to generate protective coatings. A clear understanding of the dynamic impingement when droplets make contact with substrates is critical for controlling and optimizing the thermal spray process. A droplet impingement model is developed to simulate the transient flow dynamics during impact, spreading and solidification. The volume of fluid surface tracking technique is employed within a fixed Eulerian structured mesh. The numerical model is validated with experimental data from tin droplet measurements. The results prove that thermal contact resistance is the key element in characterizing the substrate surface roughness for impingement modelling. It is found that spreading, solidification and air entrapment are closely related to surface roughness.

Siddharth Gadkari, Sai Gu, Jhuma Sadhukhan (2019)Two-dimensional mathematical model of an air-cathode microbial fuel cell with graphite fiber brush anode, In: Journal of Power Sources441227145 Elsevier

A two-dimensional mathematical model has been developed for characterizing and predicting the dynamic performance of an air-cathode MFC with graphite fiber brush used as anode. The charge transfer kinetics are coupled to the mass balance at both electrodes considering the brush anode as a porous matrix. The model has been used to study the effect of design (electrode spacing and anode size) as well as operational (substrate concentration) parameters on the MFC performance. Two-dimensional dynamic simulation allows visual representation of the local overpotential, current density and reaction rates in the brush anode and helps in understanding how these factors impact the overall MFC performance. The numerical results show that while decreasing electrode spacing and increasing initial substrate concentration both have a positive influence on power density of the MFC, reducing anode size does not affect MFC performance till almost 60 brush material has been removed. The proposed mathematical model can help guide experimental/pilot/industrial scale protocols for optimal performance.

Wei Jin, Laura Pastor- Pérez, DeKui Shen, Antonio Sepúlveda-Escribano, Sai Gu, Tomas Ramirez Reina (2018)Catalytic upgrading of biomass model compounds: Novel approaches and lessons learnt from traditional hydrodeoxygenation - a review, In: ChemCatChem11(3)pp. 924-960 Wiley

Catalytic hydrodeoxygenation (HDO) is a fundamental process for bio-resources upgrading to produce transportation fuels or added value chemicals. The bottleneck of this technology to be implemented at commercial scale is its dependence on high pressure hydrogen, an expensive resource which utilization also poses safety concerns. In this scenario, the development of hydrogen-free alternatives to facilitate oxygen removal in biomass derived compounds is a major challenge for catalysis science but at the same time it could revolutionize biomass processing technologies. In this review we have analyzed several novel approaches, including catalytic transfer hydrogenation (CTH), combined reforming and hydrodeoxygenation, metal hydrolysis and subsequent hydrodeoxygenation along with non-thermal plasma (NTP) in order to avoid the supply of external H2. The knowledge accumulated from traditional HDO sets the grounds for catalysts and processes development among the hydrogen alternatives. In this sense, mechanistic aspects for HDO and the proposed alternatives are carefully analyzed in this work. Biomass model compounds are selected aiming to provide an indepth description of the different processes and stablish solid correlations catalysts composition-catalytic performance which can be further extrapolated to more complex biomass feedstocks. Moreover, the current challenges and research trends of novel hydrodeoxygenation strategies are also presented aiming to spark inspiration among the broad community of scientists working towards a low carbon society where bio-resources will play a major role.

S Gu, CN Eastwick, KA Simmons, DG McCartney (2001)Computational fluid dynamic modelling of gas flow characteristics in a high-velocity oxy-fuel thermal spray system, In: Journal of Thermal Spray Technology10(3)pp. 461-469 Springer-Verlag

A computational fluid dynamics (CFD) model is developed to predict gas dynamic behavior in a high-velocity oxy-fuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a 12 mm combustion chamber linked to a parallel-sided nozzle. The CFD analysis is applied to investigate axisymmetric, steady-state, turbulent, compressible, and chemically combusting flow both within the gun and in a free jet region between the gun and the substrate to be coated. The combustion of oxygen and propylene is modeled using a single-step, finite-rate chemistry model that also allows for dissociation of the reaction products. Results are presented to show the effect of (1) fuel-to-oxygen gas ratio and (2) total gas flow rate on the gas dynamic behavior. Along the centerline, the maximum temperature reached is insensitive to the gas ratio but depends on the total flow. However, the value attained (∼2500 K) is significantly lower than the maximum temperature (∼3200 K) of the annular flame in the combustion chamber. By contrast, the centerline gas velocity depends on both total flow and gas ratio, the highest axial gas velocity being attained with the higher flow and most fuel-rich mixture. The gas Mach number increases through the gun and reaches a maximum value of approximately 1.6 around 5 mm downstream from the nozzle exit. The numerical calculations also show that the residual oxygen level is principally dependent on the fuel-to-oxygen ratio and decreases by approximately fivefold as the ratio is varied from 90 to 69% of the stoichiometric requirement. The CFD model is also used to investigate the effect of changes in combustion chamber size and geometry on gas dynamics, and the results are compared with the nominal 12 mm chamber baseline calculations.

H Tabbara, S Gu (2011)Numerical study of semi-molten droplet impingement, In: Applied Physics A: Materials Science & Processing104pp. 1011-1019 Springer-Verlag

Due to the low thermal conductivity of ceramics large temperature gradients are present through the powder particles during plasma spray deposition. As a result the particles often impinge at the substrate in a semi-molten form; which in turn substantially affects the final characteristics of the coating being formed. This study is dedicated to the novel modelling development and simulation of a semi-molten droplet impingement. The study examines the impingement process during impact, spreading and solidification of semi-molten zirconia. The simulation provides an insight to the heat transfer process during impact and solidification of a semi-molten powder particle and illustrates the freezing-induced break-up mechanism at the splat periphery.

Qi Zhang, L. Pastor-Perez, S. Gu, Tomas Ramirez Reina (2020)Transition Metal Carbides (TMCs) catalysts for gas phase CO2 upgrading reactions: a comprehensive overview, In: Catalysts MDPI

Increasing demand for CO2 utilization reactions and the stable character of CO2 have motivated the interest in developing highly active, selective and stable catalysts. Precious metal catalysts have been studied extensively due to their high activities, but their implementation for industrial applications is hindered due to their elevated cost. Among the materials which have comparatively low prices, transition metal carbides (TMCs) are deemed to display catalytic properties similar to Pt-group metals (Ru, Rh, Pd, Ir, Pt) in several reactions such as hydrogenation and dehydrogenation processes. In addition, they are excellent substrates to disperse metallic particles. Hence, the unique properties of TMCs make them ideal substitutes for precious metals resulting in promising catalysts for CO2 utilization reactions. This work aims to provide a comprehensive overview of recent advances on TMCs catalysts towards gas phase CO2 utilization processes, such as CO2 methanation, reverse water gas shift (rWGS) and dry reforming of methane (DRM). We have carefully analyzed synthesis procedures, performances and limitations of different TMCs catalysts. Insights on material characteristics such as crystal structure and surface chemistry and their connection with the catalytic activity are also critically reviewed.

S Gu, TJ Lu, DD Hass, HNG Wadley (2001)Thermal conductivity of zirconia coatings with zig-zag pore microstructures, In: Acta Materialia49(13, 1)pp. 2539-2547 ScienceDirect

Highly porous zirconia based thermal barrier coatings have recently been synthesised with zig-zag morphology pores which appear to impede heat flow through the thickness of the coating. A combined analytical/numerical study of heat conduction across these microstructures is presented and compared with thermal conductivity measurements. The effects of pore volume fraction, pore type, pore orientation and pore spacing, together with the wave length and the amplitude of zig-zag pore microstructures on overall thermal performance are quantified. The results indicate that even a few volume percent of zig-zag inter-column pores oriented normal to the substrate surface reduce the overall thermal conductivity of the coatings by more than 50%.

Duo Zhang, Qiong Cai, Oluwadamilola O. Taiwo, Vladimir Yufit, Nigel P Brandon, Sai Gu (2018)The effect of wetting area in carbon paper electrode on the performance of vanadium redox flow batteries: A three-dimensional lattice Boltzmann study, In: Electrochimica Acta283pp. 1806-1819 Elsevier

The vanadium redox flow battery (VRFB) has emerged as a promising technology for large-scale storage of intermittent power generated from renewable energy sources due to its advantages such as scalability, high energy efficiency and low cost. In the current study, a three-dimensional(3D) Lattice Boltzmann model is developed to simulate the transport mechanisms of electrolyte flow, species and charge in the vanadium redox flow battery at the micro pore scale. An electrochemical model using the Butler-Volmer equation is used to provide species and charge coupling at the surface of active electrode. The detailed structure of the carbon paper electrode is obtained using X-ray Computed Tomography(CT). The new model developed in the paper is able to predict the local concentration of different species, over-potential and current density profiles under charge/discharge conditions. The simulated capillary pressure as a function of electrolyte volume fraction for electrolyte wetting process in carbon paper electrode is presented. Different wet surface area of carbon paper electrode correspond to different electrolyte volume fraction in pore space of electrode. The model is then used to investigate the effect of wetting area in carbon paper electrode on the performance of vanadium redox flow battery. It is found that the electrochemical performance of positive half cell is reduced with air bubbles trapped inside the electrode.

Utsab Guharoy, Estelle Le Saché, Qiong Cai, Tomas Ramirez Reina, Sai Gu (2018)Understanding the role of Ni-Sn interaction to design highly effective CO2 conversion catalysts for dry reforming of methane, In: Journal of CO2 Utilization27pp. 1-10 Elsevier

CO2 reforming of methane is an effective route for carbon dioxide recycling to valuable syngas. However conventional catalysts based on Ni fail to overcome the stability requisites in terms of resistance to coking and sintering. In this scenario, the use of Sn as promoter of Ni leads to more powerful bimetallic catalysts with enhanced stability which could result in a viable implementation of the reforming technology at commercial scale. This paper uses a combined computational (DFT) and experimental approach, to address the fundamental aspects of mitigation of coke formation on the catalyst’s surface during dry reforming of methane (DRM). The DFT calculation provides fundamental insights into the DRM mechanism over the mono and bimetallic periodic model surfaces. Such information is then used to guide the design of real powder catalysts. The behaviour of the real catalysts mirrors the trends predicted by DFT. Overall the bimetallic catalysts are superior to the monometallic one in terms of long-term stability and carbon tolerance. In particular, low Sn concentration on Ni surface effectively mitigate carbon formation without compromising the CO2 conversion and the syngas production thus leading to excellent DRM catalysts. The bimetallic systems also presents higher selectivity towards syngas as reflected by both DFT and experimental data. However, Sn loading has to be carefully optimized since a relatively high amount of Sn can severely deter the catalytic performance.

LM Armstrong, S Gu, KH Luo, P Mahanta (2013)Multifluid Modeling of the Desulfurization Process within a Bubbling Fluidized Bed Coal Gasifier, In: AIChE Journal59(21 May)pp. 1952-1963 American Institute of Chemical Engineers

The desulfurization process to a two-dimensional (2-D) and three-dimensional (3-D) Eulerian–Eulerian computational fluid dynamic (CFD) model of a coal bubbling fluidized gasifier is introduced. The desulfurization process is important for the reduction of harmful SOx emissions; therefore, the development of a CFD model capable of predicting chemical reactions involving desulfurization is key to the optimization of reactor designs and operating conditions. To model the process, one gaseous phase and five particulate phases are included. Devolatilization, heterogeneous, and homogeneous chemical reactions as well as calcination and desulfurization reactions are incorporated. A calcination-only model and a calcination plus desulfurization model are simulated in 2-D and 3-D and the concentrations of SO2 leaving the reactors are compared. The simulated results are assessed against available published experimental data. The influence of the fluidized bed on the desulfurization is also considered.

E Gozali, M Mahrukh, S Gu, S Kamnis (2014)Numerical Analysis of Multicomponent Suspension Droplets in High-Velocity Flame Spray Process, In: Journal of Thermal Spray Technology23(6)pp. 940-949 Springer US

The liquid feedstock or suspension as a different mixture of liquid fuel ethanol and water is numerically studied in high-velocity suspension flame spray (HVSFS) process, and the results are compared for homogenous liquid feedstock of ethanol and water. The effects of mixture on droplet aerodynamic breakup, evaporation, combustion, and gas dynamics of HVSFS process are thoroughly investigated. The exact location where the particle heating is initiated (above the carrier liquid boiling point) can be controlled by increasing the water content in the mixture. In this way, the particle inflight time in the high-temperature gas regions can be adjusted avoiding adverse effects from surface chemical transformations. The mixture is modeled as a multicomponent droplet, and a convection/diffusion model, which takes into account the convective flow of evaporating material from droplet surface, is used to simulate the suspension evaporation. The model consists of several sub-models that include premixed combustion of propane-oxygen, non-premixed ethanol-oxygen combustion, modeling of multicomponent droplet breakup and evaporation, as well as heat and mass transfer between liquid droplets and gas phase.

—Multi-numerology multi-carrier (MN-MC) techniques are considered as essential enablers for RAN slicing in fifth-generation (5G) communication systems and beyond. However, utilization of mixed numerologies breaks the or-thogonality principle defined for single-numerology orthogonal frequency division multiplexing (SN-OFDM) systems with a unified subcarrier spacing. This leads to interference between different numerologies, i.e., inter-numerology interference (INI). This paper develops metrics to quantify the level of the INI using a continuous-time approach. The derived analytical expressions of INI in terms of mean square error (MSE) and error vector magnitude (EVM) directly reveal the main contributing factors to INI, which can not be shown explicitly in a matrix form INI based on discrete-time calculations. Moreover, the study of power offset between different numerologies shows a significant impact on INI, especially for high order modulation schemes. The finding in this paper provides analytical guidance in designing multi-numerology (MN) systems, for instance, developing resource allocation schemes and interference mitigation techniques.

S GU, D. G MCCARTNEY, C. N EASTWICK, K SIMMONS (2004)Numerical modeling of in-flight characteristics of Inconel 625 particles during high-velocity oxy-fuel thermal spraying, In: Journal of thermal spray technology13(2)pp. 200-213 Springer
S Gu, C N Eastwick, K A Simmons, D G McCartney (2001)Computation fluid dynamic modeling of gas flow characteristics in a high-velocity oxy-fuel thermal spray system, In: Journal of thermal spray technology10(3)pp. 461-469

A computational fluid dynamics (CFD) model is developed to predict gas dynamic behavior in a high-velocity oxy-fuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a 12 mm combustion chamber linked to a parallel-sided nozzle. The CFD analysis is applied to investigate axisymmetric, steady-state, turbulent, compressible, and chemically combusting flow both within the gun and in a free jet region between the gun and the substrate to be coated. The combustion of oxygen and propylene is modeled using a single-step, finite-rate chemistry model that also allows for dissociation of the reaction products. Results are presented to show the effect of (1) fuel-to-oxygen gas ratio and (2) total gas flow rate on the gas dynamic behavior. Along the centerline, the maximum temperature reached is insensitive to the gas ratio but depends on the total flow. However, the value attained ([similar to]2500 K) is significantly lower than the maximum temperature ([similar to]3200 K) of the annular flame in the combustion chamber. By contrast, the centerline gas velocity depends on both total flow and gas ratio, the highest axial gas velocity being attained with the higher flow and most fuel-rich mixture. The gas Mach number increases through the gun and reaches a maximum value of approximately 1.6 around 5 mm downstream from the nozzle exit. The numerical calculations also show that the residual oxygen level is principally dependent on the fuel-to-oxygen ratio and decreases by approximately fivefolds as the ratio is varied from 90 to 69% of the stoichiometric requirement. The CFD model is also used to investigate the effect of changes in combustion chamber size and geometry on gas dynamics, and the results are compared with the nominal 12 mm chamber baseline calculations.

Liuqingqing Yang, Laura Pastor Perez, J.J Villora-Pico, Sai Gu, A. Sepúlveda-Escribano, Tomas Ramirez Reina (2020)CO2 valorisation via Reverse Water-Gas Shift reaction using promoted Fe/CeO2-Al2O3 catalysts: showcasing the potential of advanced catalysts to explore new processes design, In: APPLIED CATALYSIS A-GENERAL593117442 ELSEVIER SCIENCE BV

The RWGS reaction represents a direct approach for gas-phase CO2 upgrading. This work showcases the efficiency of Fe/CeO2-Al2O3 catalysts for this process, and the effect of selected transition metal promoters such as Cu, Ni and Mo. Our results demonstrated that both Ni and Cu remarkably improved the performance of the monometallic Fe-catalyst. The competition Reverse Water-Gas Shift (RWGS) reaction/CO2 methanation reaction was evident particularly for the Ni-catalyst, which displayed high selectivity to methane in the low-temperature range. Among the studied catalysts the Cu promoted sample represented the best choice, exhibiting the best activity/selectivity balance. In addition, the Cu-doped catalyst was very stable for long-term runs – an essential requisite for its implementation in flue gas upgrading units. This material can effectively catalyse the RWGS reaction at medium-low temperatures, providing the possibility to couple the RWGS reactor with a syngas conversion reaction. Such an integrated unit opens the horizons for a direct CO2 to fuels/chemicals approach.

N Zeoli, H Tabbara, S Gu (2011)CFD modelling of primary breakup during metal powder atomization, In: Chemical Engineering Science66(24 (15)pp. 6498-6504 Elsevier B.V.

Powder metals are the basis of powder metallurgy with a large variety of applications, including sintering and thermal spray coatings. The Gas atomization process has been widely employed as an effective method to produce fine spherical metal powders. New applications and emerging surface technologies demand high quality powders with a very narrow particle size distribution. A computational fluid dynamics (CFD) approach is developed to examine complex fluids during atomization from different nozzle designs, using the volume of fluid (VOF) method and the Reynolds Stress Model (RSM). The modeling results show that the swirling gas atomizer is not beneficial to the atomization process while the inner-jet atomizer can improve the powder generation processing.

Siddharth Gadkari, B Fidalgo, Sai Gu (2016)Numerical analysis of microwave assisted thermocatalytic decomposition of methane, In: International Journal of Hydrogen Energy42(7)pp. 4061-4068 Elsevier

A comprehensive 3D coupled mathematical model is developed to study the microwave assisted thermocatalytic decomposition of methane with activated carbon as the catalyst. A simple reaction kinetic model for methane conversion (accounting for catalyst deactivation) is developed from previously published experimental data and coupled with the governing equations for the microwaves, heat transfer, mass transfer and fluid flow physics. Temperature distribution and concentration profiles of CH4 & H2 in the catalyst bed are presented. The temperature profiles at di erent input power values predict a non-uniform temperature distribution with hot-spots near the top and bottom of the catalyst. The concentration profiles predict a linear variation of CH4 and H2 concentration along the length of the reactor and show a good agreement with experimental conversion values. The influence of volumetric hourly space velocity on methane conversion is also investigated.

A two-dimensional numerical fluid model is developed for studying the influence of packing configurations on dielectric barrier discharge (DBD) characteristics. Dis- charge current profiles, and time averaged electric field strength, electron number density and electron temperature distributions are compared for the three DBD configurations, plain DBD with no packing, partially packed DBD and fully packed DBD. The results show a strong change in discharge behaviour occurs when a DBD is fully packed as compared to partial packing or no packing. While the average electric field strength and electron temperature of a fully packed DBD are higher relative to the other DBD configurations, the average electron density is substantially lower and may impede the DBD reactor performance under certain operating conditions. Possible scenarios of the synergistic effect of the combination of plasma with catalysis are also discussed.

S Kamnis, S Gu, TJ Lu, C Chen (2009)Numerical Modelling the Bonding Mechanism of HVOF Sprayed Particles, In: Computational Materials Science46(4, Oct)pp. 1038-1043 Elsevier B.V.

During high velocity oxy-fuel (HVOF) thermal spraying, most powder particles remain in solid state prior to the formation of coating. A finite element (FE) model is developed to study the impact of thermally sprayed solid particles on substrates and to establish the critical particle impact parameters needed for adequate bonding. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results indicate that in HVOF process the kinetic energy of the particle prior to impact plays the most dominant role on particle stress localization and melting of the particle/substrate interfacial region. Both the shear-instability theory and an energy-based method are used to establish the critical impact parameters for HVOF sprayed particles, and it is found that only WC-Co particles smaller than 40 μm have sufficient kinetic and thermal energy for successful bonding.

S Gu, S Kamnis (2009)Numerical Modelling of In-Flight Particle Dynamics of Non-Spherical Powder, In: Surface and Coatings Technology203(22, 15)pp. 3485-3490 Elsevier B.V.

High velocity oxygen fuel (HVOF) is an important thermal spraying technology in depositing high quality coatings. Its ability to produce high particle velocities and relatively low particle temperatures is its most salient feature. Several computational fluid dynamic (CFD) models have been developed to study the in-flight particle behavior during thermal spraying. These models are limited to spherical particles, which are only appropriate for modelling gas atomised powders. On the other hand, hardmetal powders such as WC-Co are created using high energy ball milling and are not normally spherical. To examine the effect of particle morphology on particle dynamics, mathematical models are developed in the present paper to predict the in-flight particle behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow are solved numerically. The results demonstrate that non-spherical particles gain more momentum and less heat during the HVOF process than spherical particles. Non-spherical particles are also predicted to stay closer to the center of the gas jet than spherical particles.

K Papadikis, S Gu, AV Bridgwater (2010)Geometrical Optimization of a Fast Pyrolysis Bubbling Fluidized Bed Reactor Using Comutational Fluid Dynamics, In: Energy & Fuels24(10 (20)pp. 5634-5651 ACS Publications

This paper analyzes the physical phenomena that take place inside an 1 kg/h bubbling fluidized bed reactor located at Aston University and presents a geometrically modified version of it, in order to improve certain hydrodynamic and gas flow characteristics. The bed uses, in its current operation, 40 L/min of N2 at 520 °C fed through a distributor plate and 15 L/min purge gas stream, i.e., N2 at 20 °C, via the feeding tube. The Eulerian model of FLUENT 6.3 is used for the simulation of the bed hydrodynamics, while the k − ϵ model accounts for the effect of the turbulence field of one phase on the other. The three-dimensional simulation of the current operation of the reactor showed that a stationary bubble was formed next to the feeding tube. The size of the permanent bubble reaches up to the splash zone of the reactor, without any fluidizaton taking place underneath the feeder. The gas flow dynamics in the freeboard of the reactor is also analyzed. A modified version of the reactor is presented, simulated, and analyzed, together with a discussion on the impact of the flow dynamics on the fast pyrolysis of biomass.

K Papadikis, S Gu, AV Bridgwater (2009)A CFD approach on the effect of particle size on char entrainment in bubbling fluidised bed reactors, In: Biomass and Bioenergy34(1, Jan)pp. 21-29 Elsevier B.V.

The fluid – particle interaction inside a 41.7 mg s−1 fluidised bed reactor is modelled. Three char particles of sizes 500 μm, 250 μm, and 100 μm are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions and reactor design the char particles will either be entrained from the reactor or remain inside the bubbling bed. The particle size is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. A 3-Dimensional simulation has been performed with a completele revised momentum transport model for bubble three-phase flow according to the literature as an extension to the commercial finite volume code FLUENT 6.2.

Siddharth Gadkari, Xin Tu, Sai Gu (2017)Fluid model for a partially packed dielectric barrier discharge plasma reactor, In: Physics of Plasmas24(9)pp. 093510-1 AIP Publishing

In this work, a two-dimensional numerical fluid model is developed for a partially packed dielectric barrier discharge (DBD) in pure helium. In fluence of packing on the discharge characteristics is studied by comparing the results of DBD with partial packing with those obtained for DBD with no packing. In the axial partial packing configuration studied in this work, the electric field strength was shown to be en hanced at the top surface of the spherical packing material and at the contact points between the packing and the dielectric layer. For each value of applied potential, DBD with partial packing showed an increase in the number of pulses in the current profile in the positive half cycle of the applied voltage, as compared to DBD with no packing. Addition of partial packing to the plasma-alone DBD also led to an increase in the electron and ion number densities at the moment of breakdown. The time averaged electron energy profiles showed that a much higher range of electron energy can be achieved with the use of partial packing as compared to no packing in a DBD, at the same applied power. The spatially and time averaged values over one voltage cycle also showed an increase in power density and electron energy on inclusion of partial packing in the DBD. For the applied voltage parameters studied in this work, the discharge was found to be consistently homogeneous and showed the characteristics of atmospheric pressure glow discharge.

S Gadkari, B Fidalgo, S Gu (2016)Numerical investigation of microwave-assisted pyrolysis of lignin, In: Fuel Processing Technology156pp. 473-484 Elsevier

A comprehensive three-dimensional mathematical model is developed for studying the microwave-assisted pyrolysis of biomass. Kraft Lignin is considered as biomass feedstock in the model, and a mixture of lignin and char, is used as the sample for pyrolysis. A lumped kinetic model which considers three lumped pyrolysis products (gas, liquid and remaining solid fractions) is coupled with the governing equations for the microwave field, heat transfer, mass transfer, Darcy fluid flow and a transient numerical analysis is performed. The distribution of electric field in the microwave cavity, and the distribution of electric field, temperature, and pyrolysis products within the lignin sample are presented. The lignin sample is predicted to undergo volumetric heating when subjected to microwave heating. Accordingly the reaction zone extends from the center of the biomass sample bed towards the outer surface. Preliminary evaluation of the applicability of the model for assessing the effect of different parameters on the microwave pyrolysis of lignin is also carried out.

S Kamnis, S Gu, M Vardavoulias (2010)The influence of powder porosity on the bonding mechanism at the impact of thermally sprayed solid particles, In: Metallurgical and Materials Transactions A41(13)pp. 3517-3524 Springer US

High-velocity oxy-fuel (HVOF) thermal spraying can generate dense depositions without melting the powders during spraying. Our previous study showed that most HVOF-sprayed particles are in solid state prior to impact on the substrate. The deposition of solid particles requires sufficient deformation of the particles as a result of a high impact. This report is a continuation of our previous work to study the bonding mechanism for thermally sprayed solid particles. The same hard material, WC-Co powder, is studied by considering the porosity inside the particles. The detailed deposition mechanism is examined by dynamically tracking the particle impingement using finite element analysis (FEA) models. The results confirm that the deposition of high-speed solid particles is caused mainly by the particle deformation and further implies that deformation is enhanced with increase in porosity alone. Therefore, a possible way to increase the deposition efficiency of hard cermet coating could be to use a properly designed porous powder.

MB Shemfe, B Fidalgo, S Gu (2015)Heat integration for bio-oil hydroprocessing coupled with aqueous phase steam reforming, In: Chemical Engineering Research and Design107(March)pp. 73-80 Elsevier B.V.

Optimized heat exchanger networks can improve process profitability and minimize emissions. The aim of this study is to assess the heat integration opportunities for a hypothetical bio-oil hydroprocessing plant integrated with a steam reforming process via pinch technology. The bio-oil hydroprocessing plant was developed with rate based chemical reactions using ASPEN Plus® process simulator. The base case is a 1600 kg/h bio-oil hydroprocessing plant, which is integrated with a steam reforming process of the bio-oil aqueous phase. The impact of the reformer steam to carbon ratio on energy targets was analysed, revealing that significant energy savings can be achieved at different process variations. Aspen Energy Analyzer™ was employed to design the heat exchanger network. Two heat exchanger network designs are considered. The optimum design reveals that the second hydrodeoxygenation reactor effluent can preheat the bio-oil feed with minimal capital cost implication and achieve similar energy targets compared with the alternative design. The economic and environmental implications of the two heat exchanger network designs on product value were also evaluated.

EE Ekanem, SA Nabavi, GT Vladisavljeyic, S Gu (2015)Structured Biodegradable Polymeric Microparticles for Drug Delivery Produced Using Flow Focusing Glass Microfluidic Devices, In: ACS APPLIED MATERIALS & INTERFACES7(41)pp. 23132-23143 AMER CHEMICAL SOC

Biodegradable poly(DL-lactic acid) (PLA) and poly(lactic-co-glycolic acid) (PLGA) microparticles with tunable size, shape, internal structure and surface morphology were produced by counter-current flow focusing in axisymmetric (3D) glass capillary devices. The dispersed phase was composed of 0.5-2 wt % polymer solution in a volatile organic solvent (ethyl acetate or dichloromethane) and the continuous phase was 5 wt % aqueous poly(vinyl alcohol) solution. The droplets with a coefficient of variation in dripping regime below 2.5% were evaporated to form polymeric particles with uniform sizes ranging between 4 and 30 μm. The particle microstructure and surface roughness were modified by adding nanofiller (montmorillonite nanoclay) or porogen (2-methylpentane) in the dispersed phase to form less porous polymer matrix or porous particles with golf-ball-like dimpled surface, respectively. The presence of 2-4 wt % nanoclay in the host polymer significantly reduced the release rate of paracetamol and prevented the early burst release, as a result of reduced polymer porosity and tortuous path for the diffusing drug molecules. Numerical modeling results using the volume of fluid-continuum surface force model agreed well with experimental behavior and revealed trapping of nanoclay particles in the dispersed phase upstream of the orifice at low dispersed phase flow rates and for 4 wt % nanoclay content, due to vortex formation. Janus PLA/PCL (polycaprolactone) particles were produced by solvent evaporation-induced phase separation within organic phase droplets containing 3% (v/v) PLA/PCL (30/70 or 70/30) mixture in dichloromethane. A strong preferential adsorption of Rhodamine 6G dye onto PLA was utilized to identify PLA portions of the Janus particles by confocal laser scanning microscopy (CLSM). Uniform hemispherical PCL particles were produced by dissolution of PLA domes with acetone.

K Papadikis, S Gu, AV Bridgwater (2009)Computational modelling of the impact of particle size to the heat transfer coefficient between biomass particles and a fluidised bed, In: Fuel Processing Technology91(1, Jan)pp. 68-79 Elsevier B.V.

The fluid–particle interaction and the impact of different heat transfer conditions on pyrolysis of biomass inside a 150 g/h fluidised bed reactor are modelled. Two different size biomass particles (350 μm and 550 μm in diameter) are injected into the fluidised bed. The different biomass particle sizes result in different heat transfer conditions. This is due to the fact that the 350 μm diameter particle is smaller than the sand particles of the reactor (440 μm), while the 550 μm one is larger. The bed-to-particle heat transfer for both cases is calculated according to the literature. Conductive heat transfer is assumed for the larger biomass particle (550 μm) inside the bed, while biomass–sand contacts for the smaller biomass particle (350 μm) were considered unimportant. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Biomass reaction kinetics is modelled according to the literature using a two-stage, semi-global model which takes into account secondary reactions. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of User Defined Function (UDF).

N Zeoli, H Tabbara, S Gu (2012)Three-dimensional simulation of primary break-up in a close-coupled atomizer, In: Applied Physics A: Materials Science & Processing108(4 (Sep)pp. 783-792 Springer-Verlag

Until now much of the modelling activity around close-coupled gas atomization has been mainly focused on gas-only flow with discrete phase interaction using Lagrangian-based models. However, this approach is unable to supply valuable information regarding the primary break-up mechanism of the melt being injected. Furthermore, much of existing numerical work is based on two-dimensional axisymmetric geometries, and therefore suffers the absence of three-dimensional flow features. In order to overcome these aspects the authors have carried out an analysis using a three-dimensional geometry by means of an Eulerian, Volume of Fluid, model to accurately present the early stages of melt stream behaviour at the atomizer’s melt inlet. The study investigates the mechanisms associated with primary break-up, and the results obtained highlight three modes under which a close-coupled atomizer may operate.

K Papadikis, S Gu, AV Bridgwater (2010)3D simulation of the effects of sphericity on char entrainment in fluidised beds, In: Fuel Processing Technology91(7, Jul)pp. 749-758 Elsevier B.V.

The paper presents a 3-dimensional simulation of the effect of particle shape on char entrainment in a bubbling fluidised bed reactor. Three char particles of 350 μm side length but of different shapes (cube, sphere, and tetrahedron) are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions, reactor design and particle shape the char particles will either be entrained from the reactor or remain inside the bubbling bed. The sphericity of the particles is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. The simulation has been performed with a completely revised momentum transport model for bubble three-phase flow, taking into account the sphericity factors, and has been applied as an extension to the commercial finite volume code FLUENT 6.3.

S Kamnis, S Gu, TJ Lu, C Chen (2008)Computational simulation of thermally sprayed WC–Co powder, In: Computational Materials Science43(4, Oct)pp. 1172-1182 ScienceDirect

WC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 μm in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center.

M Mahrukh, A Kumar, S Gu (2016)Effects of Angular Injection, and Effervescent Atomization on High-Velocity Suspension Flame Spray Process, In: Surface and Coatings Technology302pp. 368-382

This work presents the nanostructured coating formation using suspension thermal spraying through the HVOF torch. The nanostructured coating formation requires nanosize powder particles to be injected inside a thermal spray torch using liquid feedstock. The liquid feedstock needs to be atomized when injected into the high-velocity oxygen fuel (HVOF) torch. This paper presents the effects of angular injection and effervescent atomization of the liquid feedstock on gas and droplet dynamics, vaporization rate, and secondary breakup in the high-velocity suspension flame spray (HVSFS) process. Different angular injections are tested to obtain the optimum value of the angle of injection. Moreover, effervescent atomization technique based on twin-fluid injection has been studied to increase the efficiency of the HVSFS process. Different solid nanoparticle concentrations in suspension droplets are considered. In angular injection the droplets are injected into the core of the combustion zone; this immediately evaporates the droplets, and evaporation is completed within the torch. The value of 10°–15° is selected as the optimal angle of injection to improve the gas and droplet dynamics inside the torch, and to avoid the collision with the torch's wall. The efficiency of the effervescent atomization can be enhanced by using high gas-to-liquid mass flow rate ratio, to increase the spray cone angle for injecting the suspension liquid directly into the combustion flame. It is also found that the increment in the nanoparticle concentration has no considerable effects on the droplet disintegration process. However, the location of evaporation is significantly different for homogeneous and non-homogeneous droplets.

H Tabbara, S Gu (2012)A Study of Liquid Droplet Disintegration for the Development of Nanostructured Coatings, In: AIChE Journal58(11 (No)pp. 3533-3544

Thermal spray coatings produced from a liquid feedstock are receiving an increasing level of interest due to the advanced, nanostructured coatings which are obtainable by these processes. In this article, a high-velocity oxy-fuel (HVOF) thermal spray system is computationally investigated to make a scientific assessment of the liquid droplet behavior on injection. An existing liquid-fuelled HVOF thermal spray gun is simulated using the computational fluid dynamic approach. The steady-state gas-phase dynamics are initialized by the introduction of liquid kerosene and oxygen which react within the combustion chamber producing a realistic compressible, turbulent jet. Discrete-phase water droplets are injected at the powder injection port. On injection, the water droplets breakup and vaporize, while being entrained through the acceleration barrel of the HVOF system. The results obtained give an insight to the mechanism which control the water droplet sizes and disintegration process, and serve as a fundamental reference for future development of liquid feedstock devices.

E Gozali, M Mahrukh, S Gu, S Kamnis (2015)Numerical Investigation on Effects of Nanoparticles on Liquid Feedstock Behavior in High Velocity Oxygen Fuel (HVOF) Suspension Spraying, In: Surface and Coatings Technology280(25 Oct)pp. 370-377 ELSEVIER SCIENCE BV

Suspension feedstock in high velocity oxy-fuel flame jets has opened a new area of research with great potential for advanced coatings. Understanding the suspension behavior in such a multidisciplinary process is a key factor in producing repeatable and controllable coatings. In this study, the effects of solid nanoparticles, suspended in liquid feedstock, on suspension fragmentation, vaporization rate and gas dynamics are investigated in the High Velocity Oxygen Fuel (HVOF) suspension spraying process by numerical modeling. The model consists of several sub-models that include pre-mixed combustion of propane–oxygen, non-premixed ethanol–oxygen combustion, modeling aerodynamic droplet break-up and evaporation, heat and mass transfer between liquid droplets and gas phase. Moreover, the thermo-physical properties of suspension (mixture of solid nanoparticles and liquid solvent) are calculated from theoretical models. The results show that small droplets carrying high nanoparticle concentrations develop higher surface tension and result in less fragmentation. The recommended ethanol droplet size at high nanoparticle loadings is found to be 50 μm due to the high evaporation rate in the mid-section of the nozzle. For larger droplets, severe fragmentation occurs inside the combustion chamber (CC) while complete evaporation takes place in the free jet region outside the gun.

M Mahrukh, A Kumar, S Ali Nabavi, Sai Gu, I Sher (2017)Numerical Analysis of the Effects of using Effervescent Atomization on Solution Precursor Thermal Spraying Process, In: Industrial & Engineering Chemistry Research56(48)pp. 14231-14244 American Chemical Society

The solution precursor thermal spraying (SPTS) process is used to obtain nano-sized dense coating layers. During the SPTS process, the in situ formation of nanoparticles is mainly dependent on combustion gas-temperature, gas-pressure, gas-velocity, torch design, fuel type, and Oxygen-Fuel (O/F) mixture ratios, precursor injection feeding ratio and flow rates, properties of fuel and precursor and its concentration, and the precursor droplets fragmentation. The focus of the present work is the numerical study of atomization of pure solvent droplets streams into fine droplets spray using an effervescent twin-fluid atomizer. For better droplet disintegration appropriate atomization techniques can be used for injecting the precursor in the CH-2000 high-velocity oxygen fuel (HVOF) torch. The CFD computations of the SPTS process are essentially required because the internal flow physics of HVOF process cannot be examined experimentally. In this research for the first time, an effervescent twin-fluid injection nozzle is designed to inject the solution precursor into the HVOF torch, and the effects on the HVOF flame dynamics are analyzed. The computational fluid dynamics (CFD) modeling is performed using Linearized Instability Sheet Atomization (LISA) model and validated by the measured values of droplets size distribution at varied Gas-to- Liquid flow rate Ratios (GLR). Different nozzle diameters with varied injection parameters are numerically tested, and results are compared to observe the effects on the droplet disintegration and evaporation. It is concluded that the effervescent atomization nozzle used in the CH-2000 HVOF torch can work efficiently even with bigger exit diameters and with higher values of viscosity and surface-tension of the solution. It can generate smaller size precursor droplets (2 μm

Z Guo, S Yin, Z Qian, H Liao, S Gu (2015)Effect of the deviation of the current density profile center on the three-dimensional non-transferred arc plasma torch, In: Computers & Fluids114(2 July)pp. 163-171 Elsevier B.V.

In this study, the three-dimensional steady-state non-transferred plasma arc was investigated using computational fluid dynamics (CFD) with user defined functions (UDFs). A two-equation current density profile was developed to simulate the complex plasma flow inside the torch. The effect of the deviation distance (distance between the cathode tip center and the current density profile center) on the plasma flow features was systematically investigated for the first time. It is found that the temperature and velocity inside the plasma column reduce as the deviation distance increases, but the temperature near the arc-root attachment shows an increasing trend. Besides, it is also found that the arc length decreases with increasing the deviation distance.

S Gu, S Kamnis (2009)Bonding mechanism from the impact of thermally sprayed solid particles, In: Metallurgical and Materials Transactions A40(Novemb) Springer US

Power particles are mainly in solid state prior to impact on substrates from high velocity oxy-fuel (HVOF) thermal spraying. The bonding between particles and substrates is critical to ensure the quality of coating. Finite element analysis (FEA) models are developed to simulate the impingement process of solid particle impact on substrates. This numerical study examines the bonding mechanism between particles and substrates and establishes the critical particle impact parameters for bonding. Considering the morphology of particles, the shear-instability–based method is applied to all the particles, and the energy-based method is employed only for spherical particles. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results confirm that in the HVOF process, the kinetic energy of the particle prior to impact plays the most dominant role in particle stress localization and melting of the interfacial contact region. The critical impact parameters, such as particle velocity and temperature, are shown to be affected by the shape of particles, while higher impact velocity is required for highly nonspherical powder.

K Papadikis, S Gu, AV Bridgwater (2009)CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors: Modelling the impact of biomass shrinkage, In: Chemical Engineering Journal149(1-3, 1)pp. 417-427 Elsevier B.V.

The fluid–particle interaction and the impact of shrinkage on pyrolysis of biomass inside a 150 g/h fluidised bed reactor is modelled. Two 500 View the MathML sourcem in diameter biomass particles are injected into the fluidised bed with different shrinkage conditions. The two different conditions consist of (1) shrinkage equal to the volume left by the solid devolatilization, and (2) shrinkage parameters equal to approximately half of particle volume. The effect of shrinkage is analysed in terms of heat and momentum transfer as well as product yields, pyrolysis time and particle size considering spherical geometries. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user defined function (UDF).

Duo Zhang, K Papadikis, Sai Gu (2014)Application of a high density ratio lattice-Boltzmann model for the droplet impingement on flat and spherical surfaces, In: International Journal of Thermal Sciences84(Octobe)pp. 75-85 Elsevier B.V.

In the current study, a 3-dimensional lattice Boltzmann model which can tolerate high density ratios is employed to simulate the impingement of a liquid droplet onto a flat and a spherical target. The four phases of droplet impact on a flat surface, namely, the kinematic, spreading, relaxation and equilibrium phase, have been obtained for a range of Weber and Reynolds numbers. The predicted maximum spread factor is in good agreement with experimental data published in the literature. For the impact of the liquid droplet onto a spherical target, the temporal variation of the film thickness on the target surface is investigated. The three different temporal phases of the film dynamics, namely, the initial drop deformation phase, the inertia dominated phase and the viscosity dominated phase are reproduced and studied. The effect of the droplet Reynolds number and the target-to-drop size ratio on the film flow dynamics is investigated.

H Tabbara, S Gu, DG McCartney (2011)Computational modelling of titanium particles in warm spray, In: Computers & Fluids44(1 (May)pp. 358-368 Elsevier B.V.

A warm spray system has been computationally investigated by introducing a centrally located mixing chamber into a HVOF thermal spray gun. The effects of injecting a cooling gas on the gas and particle dynamics are examined. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust with oxygen within the combustion chamber producing a realistic compressible, supersonic and turbulent jet. The titanium powder is tracked using the Lagrangian approach including particle heating, melting and solidification. The results present an insight into the complex interrelations between the gas and particle phases, and highlight the advantage of warm spray, especially for the deposition of oxygen sensitive materials such as titanium. This work also demonstrates the effectiveness of a computational approach in aiding the development of thermal spray devices.

Weijun Li, Sai Gu, Xiangping Zhang, Tao Chen (2020)Transfer learning for process fault diagnosis: Knowledge transfer from simulation to physical processes, In: Computers & Chemical Engineering139106904 Elsevier

Deep learning has shown great promise in process fault diagnosis. However, due to the lack of sufficient labelled fault data, its application has been limited. This limitation may be overcome by using the data generated from computer simulations. In this study, we consider using simulated data to train deep neural network models. As there inevitably is model-process mismatch, we further apply transfer learning approach to reduce the discrepancies between the simulation and physical domains. This approach will allow the diagnostic knowledge contained in the computer simulation being applied to the physical process. To this end, a deep transfer learning network is designed by integrating the convolutional neural network and advanced domain adaptation techniques. Two case studies are used to illustrate the effectiveness of the proposed method for fault diagnosis: a continuously stirred tank reactor and the pulp mill plant benchmark problem.

E Gozali, S Kamnis, S Gu (2013)Numerical investigation of combustion and liquid feedstock in high velocity suspension flame spraying process, In: Surface and Coatings Technology228(15 Aug)pp. 176-186 Elsevier B.V.

Over the last decade the interest in thick nano-structured layers has been increasingly growing. Several new applications, including nanostructured thermoelectric coatings, thermally sprayed photovoltaic systems and solid oxide fuel cells, require reduction of micro-cracking, resistance to thermal shock and/or controlled porosity. The high velocity suspension flame spray (HVSFS) is a promising method to prepare advanced materials from nano-sized particles with unique properties. However, compared to the conventional thermal spray, HVSFS is by far more complex and difficult to control because the liquid feedstock phase undergoes aerodynamic break up and vaporization. The effects of suspension droplet size, injection velocity and mass flow rate were parametrically studied and the results were compared for axial, transverse and external injection. The model consists of several sub-models that include pre-mixed combustion of propane-oxygen, non-premixed ethanol–oxygen combustion, modeling aerodynamic droplet break-up and evaporation, heat and mass transfer between liquid droplets and gas phase. Thereby, the models are giving a detailed description of the relevant set of parameters and suggest a set of optimum spray conditions serving as a fundamental reference to further develop the technology.

S Kamnis, S Gu (2009)Study of in-flight and impact dynamics of non-spherical particles from HVOF guns, In: Journal of Thermal Spray Technology19(1 (Jan)pp. 31-41 Springer US

High velocity oxygen fuel thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Unlike gas atomized spherical powders, WC-CO powders form a more complex geometry. The knowledge gained from the existing spherical powders on process control and optimization may not be directly applicable to WC-Co coatings. This paper is the first to directly examine nonspherical particle in-flight dynamics and the impingement process on substrate using computational methods. Two sets of computational models are developed. First, the in-flight particles are simulated in the CFD-based combusting gas flow. The particle information prior to impact is extracted from the CFD results and implemented in a FEA model to dynamically track the impingement of particles on substrate. The morphology of particles is examined extensively including both spherical and nonspherical powders to establish the critical particle impact parameters needed for adequate bonding.

S Kamnis, S Gu, TJ Lu, C Chen (2008)Numerical Modelling of Sequential Droplet Impingements, In: Journal of Physics D: Applied Physics41(16 (20)pp. 165303-165303 IOP Publishing

During the process of thermal spray coating, molten powders are sprayed and deposited on substrates to generate protective coatings. It is essential to have a clear understanding of the physics of droplet impingement on the surface of substrates for better control of the generation of splats and the structure of coating. A numerical model is developed in this paper to simulate the dynamics of transient flow during the impingement process, including spreading, break-up, air entrapment and solidification. The computation is achieved using the technique of volume of fluid surface tracking within a fixed Eulerian structured mesh. The three-dimensional simulation is able to accurately give a demonstration of dynamic flow patterns such as the generation of fingers, satellite droplets and pores during impingement. The numerical model is validated with experimental data from the tin droplet measurement and excellent agreement is found between the simulation and the experiment.

K Papadikis, S Gu, AV Bridgwater, H Gerhauser (2009)Application of CFD to model fast pyrolysis of biomass, In: Fuel Processing Technology90(4, Apr)pp. 504-512 Elsevier B.V.

The article deals with the CFD modelling of fast pyrolysis of biomass in an Entrained Flow Reactor (EFR). The Lagrangian approach is adopted for the particle tracking, while the flow of the inert gas is treated with the standard Eulerian method for gases. The model includes the thermal degradation of biomass to char with simultaneous evolution of gases and tars from a discrete biomass particle. The chemical reactions are represented using a two-stage, semi-global model. The radial distribution of the pyrolysis products is predicted as well as their effect on the particle properties. The convective heat transfer to the surface of the particle is computed using the Ranz-Marshall correlation.

K Papadikis, AV Bridgwater, S Gu (2008)CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors, Part A: Eulerian computation of momentum transport in bubbling fluidised beds, In: Chemical Engineering Science63(16, Au)pp. 4218-4227 ScienceDirect

The fluid–particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase, according to the literature. FLUENT 6.2 has been used as the modelling framework of the simulations with a completely revised drag model, in the form of user defined function (UDF), to calculate the forces exerted on the particle as well as its velocity components. 2-D and 3-D simulations are tested and compared. The study is the first part of a complete pyrolysis model in fluidised bed reactors.

Daniel Sebastia Saez, Sai Gu, Marco Ramaioli (2017)Effect of the contact angle on the morphology, residence time distribution and mass transfer into liquid rivulets: A CFD study, In: Chemical Engineering Science176pp. 356-366 Elsevier

Droplets and rivulets over solid surfaces play an important role in a number of engineering applications. We use a Computational Fluid Dynamics model consisting in a smooth inclined plate to study the effect of the contact angle on the morphology, residence time and mass transfer into liquid rivulets. Measurements of the contact angle—using the sessile drop method—between aqueous monoethanolamine solutions and two commercial surfaces used for gas separation, are introduced as boundary condition. Reducing the contact angle from 60° to 20° flattens the rivulet, increasing the gas-liquid interface area by 85%. The cumulative residence time broadens, with an increase of 12% in τ10, and of 37% in τ90. There is consequently, a theoretical increase of 68% in the total mass transfer rate. A sensible design of the liquid-solid interaction is therefore crucial to good mass transfer performance.

S Kamnis, S Gu, M Vardavoulias (2011)Numerical study to examine the effect of porosity on in-flight particle dynamics, In: Journal of Thermal Spray Technology20(3)pp. 630-637 Springer US

High velocity oxygen fuel (HVOF) thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Powder morphology varies according to production methods while new powder manufacturing techniques produce porous powders containing air voids which are not interconnected. The porous microstructure within the powder will influence in-flight thermal and aerodynamic behavior of particles which is expected to be different from fully solid powder. This article is devoted to study the heat and momentum transfer in a HVOF sprayed WC-Co particles with different sizes and porosity levels. The results highlight the importance of thermal gradients inside the particles as a result of microporosity and how HVOF operating parameters need to be modified considering such temperature gradient.

Vasileios Katranidis, Sai Gu, Tomas Ramirez Reina, Esat Alpay, B Allcock, S Kamnis (2017)Experimental study of high velocity oxy-fuel sprayed WC-17Co coatings applied on complex geometries. Part B: Influence of kinematic spray parameters on microstructure, phase composition and decarburization of the coatings, In: Surface and Coatings Technology328pp. 499-512 Elsevier

The aim of this study is to evaluate comprehensively the effect of spray angle, spray distance and gun traverse speed on the microstructure and phase composition of HVOF sprayed WC-17 coatings. An experimental setup that enables the isolation of each one of the kinematic parameters and the systemic study of their interplay is employed. A mechanism of particle partition and WC-cluster rebounding at short distances and oblique spray angles is proposed. It is revealed that small angle inclinations benefit notably the WC distribution in the coatings sprayed at long stand-off distances. Gun traverse speed, affects the oxygen content in the coating via cumulative superficial oxide scales formed on the as-sprayed coating surface during deposition. Metallic W continuous rims are seen to engulf small splats, suggesting crystallization that occurred in-flight.

K Papadikis, S Gu, AV Bridgwater (2011)Eulerian Model for the Condensation of Pyrolysis Vapors in a Water Condenser, In: Energy and Fuels25(4)pp. 1859-1868 ASC Publications

The paper presents the simulation of the pyrolysis vapors condensation process using an Eulerian approach. The condensable volatiles produced by the fast pyrolysis of biomass in a 100 g/h bubbling fluidized bed reactor are condensed in a water cooled condenser. The vapors enter the condenser at 500 °C, and the water temperature is 15 °C. The properties of the vapor phase are calculated according to the mole fraction of its individual compounds. The saturated vapor pressure is calculated for the vapor mixture using a corresponding states correlation and assuming that the mixture of the condensable compounds behave as a pure fluid. Fluent 6.3 has been used as the simulation platform, while the condensation model has been incorporated to the main code using an external user defined function.

S Kamnis, S Gu, N Zeoli (2007)Mathematical modelling of Inconel 718 particles in HVOF thermal spraying, In: Surface and Coatings Technology202(12, 15)pp. 2715-2724 ScienceDirect

High velocity oxygen fuel (HVOF) thermal spray technology is able to produce very dense coating without over-heating powder particles. The quality of coating is directly related to the particle parameters such as velocity, temperature and state of melting or solidification. In order to obtain this particle data, mathematical models are developed to predict particle dynamic behaviour in a liquid fuelled high velocity oxy-fuel thermal spray gun. The particle transport equations are solved in a Lagrangian manner and coupled with the three-dimensional, chemically reacting, turbulent gas flow. The melting and solidification within particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight particle characteristics of Inconel 718 are studied and the effects of injection parameters on particle behavior are examined. The computational results show that the particles smaller than 10 μm undergo melting and solidification prior to impact while the particle larger than 20 μm never reach liquid state during the process.

J Bruchmüller, BGM van Wachem, S Gu, KH Luo, RC Brown (2011)Modeling the thermochemical degradation of biomass inside a fast pyrolysis fluidized bed reactor, In: AIChE Journal58(10 (Oc)pp. 3030-3042

A fast pyrolysis process in a bubbling fluidized bed has been modeled, thoroughly reproduced and scrutinized with the help of a combined Eulerian/Lagrangian simulation method. The 3-D model is compared to experimental results from a 100 g/h bubbling fluidized bed pyrolyzer including such variables as particle composition at the outlet and gas/vapor/water yields as a function of fluidization conditions, biomass moisture concentrations, and bed temperatures. Multiprocessor simulations on a high-end computer have been carried out to enable the tracking of each of the 0.8 million individual discrete sand and biomass particles, making it possible to look at accurate and detailed multiscale information (i.e., any desired particle property, trajectory, particle interaction) over the entire particle life time. The overall thermochemical degradation process of biomass is influenced by local flow and particle properties and, therefore, accurate and detailed modeling reveals unprecedented insight into such complex processes. It has been found, that the superficial fluidization velocity is important while the particle moisture content is less significant for the final bio-oil yield.

JJ Cooke, LM Armstrong, KH Luo, S Gu (2013)Adaptive Mesh Refinement of Gas-Liquid Flow on an Inclined Plane, In: Computers & Chemical Engineering60(10 Jan)pp. 297-306 Elsevier B.V.

Carbon Capture & Storage (CCS) is one of the various methods that can be used to reduce the carbon footprint of the energy sector. The efficiency with which CO2 is absorbed from flue gas using packed columns is highly dependent on the structure of the liquid films that form on the packing materials. This work examines the hydrodynamics of these liquid films using the CFD solver, OpenFOAM to solve two-phase, isothermal, non-reacting flow using the volume-of-fluid (VOF) method. Local adaptive mesh refinement (AMR) is used to ensure improved resolution of the geometrical grids at the gas–liquid interface. Comparisons are made between the solutions obtained using AMR and those obtained using highly refined static meshes. It was observed that local AMR produced results with much better correlation to experimental data.

Mobolaji Shemfe, Siddharth Gadkari, E Yu, S Rasul, K Scott, I Head, Sai Gu, Jhuma Sadhukhan (2018)Life cycle, techno-economic and dynamic simulation assessment of bioelectrochemical systems: A case of formic acid synthesis, In: Bioresource Technology255pp. 39-49 Elsevier

A novel framework integrating dynamic simulation (DS), life cycle assessment (LCA) and techno-economic assessment (TEA) of bioelectrochemical system (BES) has been developed to study for the first time wastewater treatment by removal of chemical oxygen demand (COD) by oxidation in anode and thereby harvesting electron and proton for carbon dioxide reduction reaction or reuse to produce products in cathode. Increases in initial COD and applied potential increase COD removal and production (in this case formic acid) rates. DS correlations are used in LCA and TEA for holistic performance analyses. The cost of production of HCOOH is €0.015–0.005g–1 for its production rate of 0.094–0.26kgyr–1 and a COD removal rate of 0.038–0.106kgyr–1. The life cycle (LC) benefits by avoiding fossil-based formic acid production (93%) and electricity for wastewater treatment (12%) outweigh LC costs of operation and assemblage of BES (–5%), giving a net 61MJkg-1HCOOH saving.

Ioannis Kalargaris, Guohong Tian, Sai Gu (2017)Influence of advanced injection timing and fuel additive on combustion, performance and emission characteristics of a DI diesel engine running on plastic pyrolysis oil, In: Journal of Combustion20173126342 Hindawi Publishing Corporation

This paper presents the investigation of engine optimisation when plastic pyrolysis oil (PPO) is used as the primary fuel of a direct injection diesel engine. Our previous investigation revealed that PPO is a promising fuel however the results suggested that control parameters should be optimised in order to obtain a better engine performance. In the present work, the injection timing was advanced, and fuel additives were utilised to overcome the issues experienced in the previous work. In addition, spray characteristics of PPO were investigated in comparison with diesel to provide in-depth understanding of the engine behaviour. The experimental results on advanced injection timing (AIT) showed a reduced brake thermal efficiency and increased carbon monoxide, unburned hydrocarbons and nitrogen oxides emissions in comparison to standard injection timing. On the other hand, the addition of fuel additive resulted in a higher engine efficiency and lower exhaust emissions. Finally, the spray tests revealed that the spray tip penetration for PPO is faster than diesel. The results suggested that AIT is not a preferable option while fuel additive is a promising solution for long-term use of PPO in diesel engines.

DK Shen, S Gu, KH Luo, AV Bridgwater, MX Fang (2008)Kinetic study on thermal decomposition of woods in oxidative environment, In: Fuel88(6, Jun)pp. 1024-1030 Elsevier B.V.

The purpose of this work is to gain knowledge on kinetics of biomass decomposition under oxidative atmospheres, mainly examining effect of heating rate on different biomass species. Two sets of experiments are carried out: the first set of experiments is thermal decomposition of four different wood particles, namely aspens, birch, oak and pine under an oxidative atmosphere and analysis with TGA; and the second set is to use large size samples of wood under different heat fluxes in a purpose-built furnace, where the temperature distribution, mass loss and ignition characteristics are recorded and analyzed by a data post-processing system. The experimental data is then used to develop a two-step reactions kinetic scheme with low and high temperature regions while the activation energy for the reactions of the species under different heating rates is calculated. It is found that the activation energy of the second stage reaction for the species with similar constituent fractions tends to converge to a similar value under the high heating rate.

L. Pastor-Pérez, F. Baibars, E. Le Sache, Harvey Arellano-Garcia, Sai Gu, Tomas Ramirez Reina (2017)CO2 valorisation via Reverse Water-Gas Shift reaction using advanced Cs doped Fe-Cu/Al2O3 catalysts, In: Journal of CO2 Utilization21pp. 423-428 Elsevier

This paper evidences the viability of chemical recycling of CO2 via reverse water-gas shift reaction using advanced heterogeneous catalysts. In particular, we have developed a multicomponent Fe-Cu-Cs/Al2O3 catalyst able to reach high levels of CO2 conversions and complete selectivity to CO at various reaction conditions (temperature and space velocities). In addition, to the excellent activity, the novel-Cs doped catalyst is fairly stable for continuous operation which suggests its viability for deeper studies in the reverse water-gas shift reaction. The catalytic activity and selectivity of this new material have been carefully compared to that of Fe/Al2O3, Fe-Cu/Al2O3 and Fe-Cs/Al2O3 in order to understand each active component’s contribution to the catalyst’s performance. This comparison provides some clues to explain the superiority of the multicomponent Fe-Cu-Cs/Al2O3 catalyst

ARK Gollakota, MD Subramanyam, N Kishore, S Gu (2015)CFD simulations on the effect of catalysts on the hydrodeoxygenation of bio-oil, In: RSC Advances(52)pp. 41855-41866

Bio-oil derived from lignocellulose biomass is an emerging alternative resource to conventional fossil fuel. However, the as-obtained unprocessed bio oil is oxy-rich, has low pH and contains high moisture, which suppresses the heating value; thus, its mixing with conventional fuel is not compatible. Therefore, studies on the upgradation of bio oil using catalytic hydrodeoxygenation (HDO) have become prominent in recent years. This study presents computational fluid dynamics (CFD) based simulation results on the effect of catalysts (Pt/Al2O3, Ni–Mo/Al2O3, Co–Mo/Al2O3) on the upgradation of bio oil using a hydrodeoxygenation process in an ebullated bed reactor. These numerical simulations are performed using an Eulerian multiphase flow module that is available in a commercial CFD based solver, ANSYS Fluent 14.5. Prior to obtaining the new results, the present numerical solution methodology is validated by reproducing some of the experimental results on the upgradation of bio oil available in the literature. Furthermore, the influence of weight hourly space velocities (WHSVs), operating temperature, and pressure inside the reactor for the different catalysts on the performance of HDO for bio oil upgradation in an ebullated bed reactor are delineated. It is observed that the gaseous stream products are higher in the presence of Pt/Al2O3 catalyst; phenols are higher when Ni–Mo/Al2O3 is used, and higher aromatics are obtained with the Co–Mo/Al2O3 catalyst. Finally, a comparison among the mass fraction of the individual species of three phases with respect to different catalysts for various combinations of WHSV, temperature and pressure values are presented.

K Papadikis, S Gu, AV Bridgwater (2008)CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass transport in bubbling fluidised beds, In: Chemical Engineering Science64(5, Mar)pp. 1036-1045 ScienceDirect

The fluid–particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the heat, momentum and mass transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user-defined function (UDF). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.

K Papadikis, H Gerhauser, AV Bridgwater, S Gu (2008)CFD modelling of the fast pyrolysis of an in-flight cellulosic particle subjected to convective heat transfer, In: Biomass and Bioenergy33(1, Jan)pp. 97-107 ScienceDirect

The pyrolysis of a freely moving cellulosic particle inside a View the MathML source continuously fed fluid bed reactor subjected to convective heat transfer is modelled. The Lagrangian approach is adopted for the particle tracking inside the reactor, while the flow of the inert gas is treated with the standard Eulerian method for gases. The model incorporates the thermal degradation of cellulose to char with simultaneous evolution of gases and vapours from discrete cellulosic particles. The reaction kinetics is represented according to the Broido–Shafizadeh scheme. The convective heat transfer to the surface of the particle is solved by two means, namely the Ranz–Marshall correlation and the limit case of infinitely fast external heat transfer rates. The results from both approaches are compared and discussed. The effect of the different heat transfer rates on the discrete phase trajectory is also considered.

J Bruchmüller, S Gu, KH Luo, BGM van Wachem (2010)Discrete Element Method for Multiscale Modelling, In: Journal of Multiscale Modelling02(01n02,) World Scientific

A discrete element method (DEM) has been developed to provide highly accurate and detailed predictions of the Lagrangian particle phase. Especially in this study, DEM has been used together with an Eulerian approach for the fluid phase to look at interphase exchange phenomena in a multiphase-multiscale modeling approach. The drying process inside a fluidized bed coffee bean roaster has been chosen. Herein, heat, mass, and momentum transport are solved on a fluid cell level; heat, mass, and momentum transfer coefficients are solved at a particle scale level; and 1D temperature and moisture content profiles are solved inside each coffee bean on a sub-particle scale level. Therefore, this multiscale approach provides much more information compared to existing coffee bean roaster models. In this work, a detailed description of this method is provided and results on different scale levels have been discussed. Modeling data and experimental results are compared and found to be in good agreement.

LM Armstrong, S Gu, KH Luo (2013)Dry pressure drop prediction within Montz-Pak B1-250.45 packing with varied inclination angles and geometries, In: Industrial and Engineering Chemistry Research52(11)pp. 4372-4378

Structured packings are used to increase the surface area and promote gas–liquid contact in many chemical processes, including carbon capture. Computational fluid dynamics and performance prediction methods have the ability to aid the optimization of the structured packing designs to aid the heat and mass transfer while minimizing the pressure drop. The present work introduces pressure drop correlations that determine frictional pressure loss between Montz-Pak B1-250.45 structured packing sheets based on the inclination angle and channel geometry of the sheets. CFD simulations are carried out on the packing and are validated against published experimental data.

Hua Wang, Sai Gu, Tao Chen (2020)Experimental Investigation of the Impact of CO, C₂H₆, and H₂ on the Explosion Characteristics of CH₄, In: ACS Omega5(38)pp. 24684-24692 American Chemical Society

Gas explosions are destructive disasters in coal mines. Coal mine gas is a multi-component gas mixture, with methane (CH₄) being the dominant constituent. Understanding the process and mechanism of mine gas explosions is of critical importance to the safety of mining operations. In this work, three flammable gases (CO, C₂H₆, and H₂) which are commonly present in coal mines were selected to explore how they affect a methane explosion. The explosion characteristics of the flammable gases were investigated in a 20 L spherical closed vessel. Experiments on binary- (CH₄/CO, CH₄/C₂H₆, and CH₄/H₂) and multicomponent (CH₄/CO/C₂H₆/H₂) mixtures indicated that the explosion of such mixtures is more dangerous and destructive than that of methane alone in air, as measured by the explosion pressure. Furthermore, a self-promoting microcirculation reaction network is proposed to help analyze the chemical reactions involved in the multicomponent (CH₄/CO/C₂H₆/H₂) gas explosion. This work will contribute to a better understanding of the explosion mechanism of gas mixtures in coal mines and provide a useful reference for determining the safety limits in practice.

Qi Zhang, Laura Pastor Perez, Wei Jin, Sai Gu, Tomas Ramirez Reina (2019)Understanding the promoter effect of Cu and Cs over highly effective -Mo2C catalysts for the reverse water-gas shift reaction, In: Applied Catalysis B: Environmental244pp. 889-898 Elsevier

Mo2C is an effective catalyst for chemical CO2 upgrading via reverse water-gas shift (RWGS). In this work, we demonstrate that the activity and selectivity of this system can be boosted by the addition of promoters such as Cu and Cs. The addition of Cu incorporates extra active sites such as Cu+ and Cu0 which are essential for the reaction. Cs is an underexplored dopant whose marked electropositive character generates electronic perturbations on the catalyst’s surface leading to enhanced catalytic performance. Also, the Cs-doped catalyst seems to be in-situ activated due to a re-carburization phenomenon which results in fairly stable catalysts for continuous operations. Overall, this work showcases a strategy to design highly efficient catalysts based on promoted β-Mo2C for CO2 recycling via RWGS.

A Kumar, S Gu, S Kamnis (2012)Simulation of impact of a hollow droplet on a flat surface, In: Applied Physics A: Materials Science & Processing109(1)pp. 101-109 Springer-Verlag

Despite many theoretical and experimental works dealing with the impact of dense continuous liquid droplets on a flat surface, the dynamics of the impact of hollow liquid droplets is not well addressed. In an effort to understand dynamics of the hollow droplet impingement, a numerical study for the impact of a hollow droplet on a flat surface is presented. The impingement model considers the transient flow dynamics during impact and spreading of the droplet using the volume of fluid surface tracking method (VOF) coupled with the momentum transport model within a one-domain continuum formulation. The model is used to simulate the hydrodynamic behaviour of the impact of glycerin hollow droplet. It is found that the impact and spreading of the hollow droplet on a flat surface is distinctly different from the conventional dense droplet and has some new hydrodynamic features. A phenomenon of formation of a central counter jet of the liquid is predicted. With the help of simulations the cause of this phenomenon is discussed. Comparison of the predicted length of the central counter jet and the velocity of the counter jet front shows good agreements with the experimental data. The influence of the droplet initial impact velocity and the hollow droplet shell thickness on the impact behaviour is highlighted.

Duo Zhang, Antoni Forner-Cuenca, Oluwadamilola O. Taiwo, Vladimir Yufit, Fikile R. Brushett, Nigel P. Brandon, Sai Gu, Qiong Cai (2020)Understanding the role of the porous electrode microstructure in redox flow battery performance using an experimentally validated 3D pore-scale lattice Boltzmann model, In: Journal of Power Sources447227249 Elsevier

The porous structure of the electrodes in redox flow batteries (RFBs) plays a critical role in their performance. We develop a framework for understanding the coupled transport and reaction processes in electrodes by combining lattice Boltzmann modelling (LBM) with experimental measurement of electrochemical performance and X-ray computed tomography (CT). 3D pore-scale LBM simulations of a non-aqueous RFB are conducted on the detailed 3D microstructure of three different electrodes (Freudenberg paper, SGL paper and carbon cloth) obtained using X-ray CT. The flow of electrolyte and species within the porous structure as well as electrochemical reactions at the interface between the carbon fibers of the electrode and the liquid electrolyte are solved by a lattice Boltzmann approach. The simulated electrochemical performances are compared against the experimental measurements with excellent agreement, indicating the validity of the LBM simulations for predicting the RFB performance. Electrodes featuring one single dominant peak (i.e., Freudenberg paper and carbon cloth) show better electrochemical performance than the electrode with multiple dominant peaks over a wide pore size distribution (i.e., SGL paper), whilst the presence of a small fraction of large pores is beneficial for pressure drop. This framework is useful to design electrodes with optimal microstructures for RFB applications.

Xinye Shao, Zhili Sun, Mingchuan Yang, Sai Gu, Qing Guo (2019)NOMA-Based Irregular Repetition Slotted ALOHA for Satellite Networks, In: IEEE Communications Letters Institute of Electrical and Electronics Engineers (IEEE)

In this letter, a non-orthogonal multiple access (NOMA) scheme is employed for irregular repetition slotted ALOHA (IRSA). Specifically, packet replicas are transmitted with discrete power levels which are pre-determined by the NOMA scheme. In this case, most packet collisions can be resolved in the power domain, contributing to a much lower packet loss rate. Density evolution (DE) analysis is formulated and the degree distributions are optimized for different number of power levels. Simulation results validate our analysis and show that the proposed scheme can outperform existing IRSA schemes.

VS Kiran Kumar Palla, K Papadikis, S Gu (2015)A numerical model for the fractional condensation of pyrolysis vapours, In: Biomass and Bioenergy74(March)pp. 180-192 Elsevier B.V.

Experimentation on the fast pyrolysis process has been primarily focused on the pyrolysis reactor itself, with less emphasis given to the liquid collection system (LCS). More importantly, the physics behind the vapour condensation process in LCSs has not been thoroughly researched mainly due to the complexity of the phenomena involved. The present work focusses on providing detailed information of the condensation process within the LCS, which consists of a water cooled indirect contact condenser. In an effort to understand the mass transfer phenomena within the LCS, a numerical simulation was performed using the Eulerian approach. A multiphase multi-component model, with the condensable vapours and non-condensable gases as the gaseous phase and the condensed bio-oil as the liquid phase, has been created. Species transport modelling has been used to capture the detailed physical phenomena of 11 major compounds present in the pyrolysis vapours. The development of the condensation model relies on the saturation pressures of the individual compounds based on the corresponding states correlations and assuming that the pyrolysis vapours form an ideal mixture. After the numerical analysis, results showed that different species condense at different times and at different rates. In this simulation, acidic components like acetic acid and formic acids were not condensed as it was also evident in experimental works, were the pH value of the condensed oil is higher than subsequent stages. In the future, the current computational model can provide significant aid in the design and optimization of different types of LCSs.

Daniel Sebastia Saez, Sai Gu, Panneerselvam Ranganathan, Konstantinos Papadikis (2014)Micro-scale CFD study about the influence of operative parameters on physical mass transfer within structured packing elements, In: International Journal of Greenhouse Gas Control28(Septem)pp. 180-188 Elsevier B.V.

In this work a VOF-based 3D numerical model is developed to study the influence of several operative parameters on the gas absorption into falling liquid films. The parameters studied are liquid phase viscosity, gas phase pressure and inlet configuration, liquid–solid contact angle and plate texture. This study aims to optimize the post-combustion CO2 capture process within structured packed columns. Liquid phase viscosity is modified via MEA (i.e. monoethanolamine) concentration. The results show that an increase in liquid viscosity reduces the diffusivity of oxygen within the liquid film thus hindering the efficiency of the process. Higher pressure carries an absorption improvement that can be attractive to be applied in industry. The simulations show that enhanced oxygen absorption rates can be achieved depending on the velocity of the gas phase and the flow configuration (i.e. co- and counter-current). Also, the importance of wetting liquid–solid contact angles (i.e. less than 90°) is highlighted. Poor liquid–solid adhesion has similar effects as surface tension in terms of diminishing the spreading of the liquid phase over the metallic plate. Finally the influence of a certain geometrical pattern in the metallic surface is also assessed.

H Jahangiri, J Bennett, P Mahjoubi, K Wilson, S Gu (2014)A review of advanced catalyst development for Fischer–Tropsch synthesis of hydrocarbons from biomass derived syn-gas, In: Catalysis Science and Technology4(8)pp. 2210-2229

Fischer–Tropsch synthesis (FTS) is a process which converts syn-gas (H2 and CO) to synthetic liquid fuels and valuable chemicals. Thermal gasification of biomass represents a convenient route to produce syn-gas from intractable materials particularly those derived from waste that are not cost effective to process for use in biocatalytic or other milder catalytic processes. The development of novel catalysts with high activity and selectivity is desirable as it leads to improved quality and value of FTS products. This review paper summarises recent developments in FT-catalyst design with regards to optimising catalyst activity and selectivity towards synthetic fuels.

Daniel Sebastia Saez, Sai Gu, Panneerselvam Ranganathan, Konstantinos Papadikis (2013)3D modeling of hydrodynamics and physical mass transfer characteristics of liquid film flows in structured packing elements, In: International Journal of Greenhouse Gas Control19(Novemb)pp. 492-502 Elsevier B.V.

Post-combustion CO2 capture by chemical absorption in structured packed columns has been technically and commercially proven as a viable option to be deployed for carbon emissions mitigation. In this work, a three dimensional CFD model at small scale for hydrodynamics and physical mass transfer in structured packing elements is developed. The results from the present model are validated with theory and reported experimental data. For hydrodynamics, the liquid film thickness and wetted area are calculated whereas for mass transfer, the Sherwood number and concentrations of dissolved species are predicted. The CFD results match reasonably with experimental and theoretical data. Furthermore, the influence of texture patterns and the liquid phase viscosity on the wetted area is studied. It is found that both parameters have a strong influence on the results. For physical mass transfer, the study of the transient behavior and the impact of the liquid load on the absorption rate is assessed. It is observed that lower liquid loads maximize mass transfer coefficients but also enhance liquid misdistribution (i.e. with the possibility of hindering mass transfer). An optimum liquid load is found where the effect of liquid misdistribution can be avoided, maximizing gas absorption.

Daniel Sebastia Saez, Sai Gu, Panneerselvam Ranganathan, Konstantinos Papadikis (2015)Meso-scale CFD study of the dry pressure drop, liquid hold-up, interfacial area and mass transfer of structured packing materials, In: International Journal of Greenhouse Gas Control42(Novemb)pp. 388-399 Elsevier B.V.

This work presents a meso-scale CFD methodology to describe the multiphase flow inside commercial structured packings for post-combustion CO2 capture. Meso-scale simulations of structured packings are often limited in the literature to dry pressure drop analyses whereas mass transfer characteristics and gas–liquid interface tracking are usually investigated at micro-scale. This work aims at testing further capabilities of meso-scale modeling by implementing the interface tracking instead of analyzing only the dry pressure drop performance with single-phase simulations. By doing so, it is possible to present also the hydrodynamics (i.e. liquid hold-up and interfacial area) for a small set of representative elementary units (REUs). The interest in interface tracking using commercial geometries lies on the fact that liquid hold-up and interfacial area have implications of capital importance on the overall performance of the absorber, hence the importance of developing a model to predict them accurately. The results show how the relationship, reported in the literature, between the liquid load and both the liquid hold-up and the interfacial area is reproduced by the present CFD methodology. Also, a more realistic visualization is accomplished with images of the inner irregularities of the flow (i.e. liquid maldistribution, formation of droplets and rivulets, etc.), which lie far from the prevailing assumption of the formation of a perfectly developed liquid film over the packing. Moreover, the effect of operating parameters such as the liquid load, liquid viscosity and liquid–solid contact angle on the amount of interfacial area available for mass transfer is also discussed. Finally, mass source terms are also included to describe the gas absorption into the liquid phase hence testing all the capabilities of micro-scale modeling at meso-scale. The present model could be further used for the analysis and optimization of other structured packing geometries.

VS Kiran Kumar Palla, K Papadikis, S Gu (2014)Computational modelling of the condensation of fast pyrolysis vapours in a quenching column. Part A: Hydrodynamics, heat transfer and design optimisation, In: Fuel Processing Technology131(March)pp. 59-68 Elsevier B.V.

Direct contact heat exchangers (quenching columns) are considered to be the optimum types of heat exchangers for the fast pyrolysis process. In this study, the hydrodynamics and heat transfer characteristics of a bench scale quenching column are presented. These have been compared with the experimental observations on flooding phenomena which are reported when the quenching column is operated at the design gas flow rates of the fast pyrolysis reactor. The quenching column was found to operate without flooding at 10% of the design flow rate, while flooding was still present even at 50% of the design gas flow rate. Four different design configurations, which are different in terms of weirs and hole placement on the disc and donut plates, are modelled and tested under full gas flow rate conditions. All four cases show normal quenching column operation without any flooding phenomena present and a gas flow time of less than 1 s. The pressure drop across the system was considerably reduced to 15Pa in the modified configuration compared to 90Pa in the baseline model. The hydrodynamic and heat transfer characteristics are thoroughly analysed and proposed optimal design configuration for the effective quenching operation.

Weijun Li, Sai Gu, Xiangping Zhang, Tao Chen (2020)A pattern matching and active simulation method for process fault diagnosis, In: Industrial & Engineering Chemistry Research American Chemical Society

Fault detection and diagnosis is a crucial approach to ensure safe and efficient operation of chemical processes. This paper reports a new fault diagnosis method that exploits dynamic process simulation and pattern matching techniques. The proposed method consists of a simulated fault database which, through pattern matching, helps narrow down the fault candidates in an efficient way. An optimisation based fault reconstruction method is then developed to determine the fault pattern from the candidates, and the corresponding magnitude and time of occurrence of the fault. A major advantage of this approach is capable of diagnosing both single and multiple faults. We illustrate the effectiveness of the proposed method through case studies of the Tennessee Eastman benchmark process.

S.Y. Liu, D.H. Mei, M.A. Nahil, Siddharth Gadkari, Sai Gu, P.T. Williams, X. Tu (2017)Hybrid plasma-catalytic steam reforming of toluene as a biomass tar model compound over Ni/Al 2 O 3 catalysts, In: Fuel Processing Technology166pp. 269-275 Elsevier

In this study, plasma-catalytic steam reforming of toluene as a biomass tar model compound was carried out in a coaxial dielectric barrier discharge (DBD) plasma reactor. The effect of Ni/Al2O3 catalysts with different nickel loadings (5–20 wt%) on the plasma-catalytic gas cleaning process was evaluated in terms of toluene conversion, gas yield, by-products formation and energy efficiency of the plasma-catalytic process. Compared to the plasma reaction without a catalyst, the combination of DBD with the Ni/Al2O3 catalysts significantly enhanced the toluene conversion, hydrogen yield and energy efficiency of the hybrid plasma process, while significantly reduced the production of organic by-products. Increasing Ni loading of the catalyst improved the performance of the plasma-catalytic processing of toluene, with the highest toluene conversion of 52% and energy efficiency of 2.6 g/kWh when placing the 20 wt% Ni/Al2O3 catalyst in the plasma. The possible reaction pathways in the hybrid plasma-catalytic process were proposed through the combined analysis of both gas and liquid products.

LM Armstrong, S Gu, KH Luo (2011)Effects of limestone calcination on the gasification processes in a BFB coal gasifier, In: Chemical Engineering Journal168(2, (1)pp. 848-860 Elsevier B.V.

An Eulerian–Eulerian computational fluid dynamics (CFD) model of the gasification processes in a coal bubbling fluidised bed (BFB) is presented incorporating the devolatilisation, heterogeneous, homogeneous reactions and limestone calcination. The model considers separate phases for the coal, limestone and char and is carried out for different experimental conditions taken from the literature. The results of the exiting gas compositions have been averaged over time and validated with experimental data. The hydrodynamic behaviour as well as temperature and reaction distributions within the bed is presented. The impact of limestone calcination on the gaseous composition is observed.

Tomas Ramirez Reina, Utsab Guharoy, Emilia Olsson, Sai Gu, Qiong Cai (2019)Theoretical insights of Ni2P (0001) surface towards its potential applicability in CO2 conversion via dry reforming of methane, In: ACS Catalysis9(4)pp. 3487-3497 American Chemical Society

This study reports the potential application of Ni2P as highly effective catalyst for chemical CO2 recycling via dry reforming of methane (DRM). Our DFT calculations reveal that the Ni2P (0001) surface is active towards adsorption of the DRM species, with the Ni hollow site being the most energetically stable site and Ni-P and P contributes as co-adsorption sites in DRM reaction steps. Free energy analysis at 1000 K found CH-O to be the main pathway for CO formation. The competition of DRM and reverse water gas shift (RWGS) is also evidenced with the latter becoming important at relatively low reforming temperatures. Very interestingly oxygen seems to play a key role in making this surface resistant towards coking. From microkinetic analysis we have found greater oxygen surface coverage than that of carbon at high temperatures which may help to oxidize carbon deposits in continuous runs. The tolerance of Ni2P towards carbon deposition was further corroborated by DFT and micro kinetic analysis. Along with the higher probability of C oxidation we identify poor capacity of carbon diffusion on the Ni2P (0001) surface with very limited availability of C nucleation sites. Overall, this study opens new avenues for research in metal-phosphide catalysis and expands the application of these materials to CO2 conversion reactions.

ARK Gollakota, N Kishore, Sai Gu (2017)A Review on Hydrothermal Liquefaction of Biomass, In: Renewable and Sustainable Energy Reviews81(1)pp. 1378-1392 Elsevier

The rapid depletion of conventional fossil fuels and day-by-day growth of environmental pollution due to use of extensive use of fossil fuels have raised concerns over the use of the fossil fuels; and thus search for alternate renewable and sustainable sources for fuels has started in the last few decades. In this context biomass derived fuels seems to be the promising path; and various routes are available for the biomass processing such as pyrolysis, transesterification, hydrothermal liquefaction, steam reforming, etc.; and the hydrothermal liquefaction (HTL) of wet biomass seems to be the promising route. Therefore, this article briefly enlightened a few concepts of HTL such as the elemental composition of bio-crude obtained by HTL, different types of feedstock adopted for HTL, mechanism of HTL processes, possible process flow diagrams for HTL of both wet and dry biomass and energy efficiency of the process. In addition, this article also enlisted possible future research scope for concerned researchers and a few of them are setting up HTL plant suitable for both wet and dry biomass feedstock; analysing influence of parameters such as temperature, pressure, residence time, catalytic effects, etc.; deriving optimized pathways for better conversion; and development of theoretical models representing the process to the best possible accuracy depending on nature of feedstock.

K Papadikis, S Gu, A Fivga, AV Bridgwater (2010)Numerical comparison of the drag models of granular flows applied to the fast pyrolysis of biomass, In: Energy and Fuels24(3)pp. 2133-2145 ACS Publications

The paper presents a comparison between the different drag models for granular flows developed in the literature and the effect of each one of them on the fast pyrolysis of wood. The process takes place on an 100 g/h lab scale bubbling fluidized bed reactor located at Aston University. FLUENT 6.3 is used as the modeling framework of the fluidized bed hydrodynamics, while the fast pyrolysis of the discrete wood particles is incorporated as an external user defined function (UDF) hooked to FLUENT’s main code structure. Three different drag models for granular flows are compared, namely the Gidaspow, Syamlal O’Brien, and Wen-Yu, already incorporated in FLUENT’s main code, and their impact on particle trajectory, heat transfer, degradation rate, product yields, and char residence time is quantified. The Eulerian approach is used to model the bubbling behavior of the sand, which is treated as a continuum. Biomass reaction kinetics is modeled according to the literature using a two-stage, semiglobal model that takes into account secondary reactions.

D Zhang, K Papadikis, S Gu (2014)Three-dimensional multi-relaxation time lattice-Boltzmann model for the drop impact on a dry surface at large density ratio, In: International Journal of Multiphase Flow64(Septem)pp. 11-18 Elsevier B.V.

Extensive application of the multiphase lattice Boltzmann model to realistic fluid flows is often restricted by the numerical instabilities induced at high liquid-to-gas density ratios, and at low viscosities. In this paper, a three-dimensional multi-relaxation time (MRT) lattice Boltzmann model with an improved forcing scheme is reported for simulating multiphase flows at high liquid-to-gas density ratios and relatively high Reynolds numbers. The model is based on a recently presented model in the literature. Firstly, the MRT multiphase model is evaluated by verifying Laplace’s law and achieving thermodynamic consistency for a static droplet. Then, a relationship between the fluid–solid interaction potential parameter and contact angle is investigated. Finally, the improved three-dimensional MRT Lattice Boltzmann model is employed in the simulation of the impingement of a liquid droplet onto a flat surface for a range of Weber and Reynolds numbers. The dynamics of the droplet spreading is reproduced and the predicted maximum spread factor is in good agreement with experimental data published in the literature.

Vasileios Katranidis, Sai Gu, Tomas Ramirez Reina, Esat Alpay, Bryan Allcock, Spyros Kamnis (2017)Experimental study of high velocity oxy-fuel sprayed WC-17Co coatings applied on complex geometries. Part B: Influence of kinematic spray parameters on microstructure, phase composition and decarburization of the coatings, In: Surface & Coatings Technology328pp. 499-512 Elsevier

The aim of this study is to evaluate comprehensively the effect of spray angle, spray distance and gun traverse speed on the microstructure and phase composition of HVOF sprayed WC-17 coatings. An experimental setup that enables the isolation of each one of the kinematic parameters and the systemic study of their interplay is employed. A mechanism of particle partition and WC-cluster rebounding at short distances and oblique spray angles is proposed. It is revealed that small angle inclinations benefit notably the WC distribution in the coatings sprayed at long stand-off distances. Gun traverse speed, affects the oxygen content in the coating via cumulative superficial oxide scales formed on the as-sprayed coating surface during deposition. Metallic W continuous rims are seen to engulf small splats, suggesting crystallization that occurred in-flight.

Weijun Li, Hui Li, Sai Gu, Tao Chen (2020)Process fault diagnosis with model- and knowledge-based approaches: Advances and opportunities, In: Control Engineering Practice105104637 Elsevier

Fault diagnosis plays a vital role in ensuring safe and efficient operation of modern process plants. Despite the encouraging progress in its research, developing a reliable and interpretable diagnostic system remains a challenge. There is a consensus among many researchers that an appropriate modelling, representation and use of fundamental process knowledge might be the key to addressing this problem. Over the past four decades, different techniques have been proposed for this purpose. They use process knowledge from different sources, in different forms and on different details, and are also named model-based methods in some literature. This paper first briefly introduces the problem of fault detection and diagnosis, its research status and challenges. It then gives a review of widely used model- and knowledge-based diagnostic methods, including their general ideas, properties, and important developments. Afterwards, it summarises studies that evaluate their performance in real processes in process industry, including the process types, scales, considered faults, and performance. Finally, perspectives on challenges and potential opportunities are highlighted for future work.

H Tabbara, S Gu, DG McCartney, TS Price, PH Shipway (2010)Study on process optimisation of cold gas spraying, In: Journal of Thermal Spray Technology20(3)pp. 608-620 Springer US

Cold gas dynamic spraying is a relatively new spray coating technique capable of depositing a variety of materials without extensive heating. As a result the inherent degradation of the powder particles found during traditional thermal spraying can be avoided. The simplicity of this technique is its most salient feature. High pressure gas is accelerated through a convergent-divergent nozzle up to supersonic velocity. The powder particles are carried to the substrate by the gas and on impact the particles deform at temperatures below their melting point. Computational modeling of thermal spray systems can provide thorough descriptions of the complex, compressible, particle-laden flow, and therefore can be utilized to strengthen understanding and allow technological progress to be made in a more systematic fashion. The computational fluid dynamic approach is adopted in this study to examine the effects of changing the nozzle cross-section shape, particle size and process gas type on the gas flow characteristics through a cold spray nozzle, as well as the spray distribution and particle velocity variation at the exit.

Z Guo, S Yin, H Liao, S Gu (2014)Three-dimensional simulation of an argon–hydrogen DC non-transferred arc plasma torch, In: International Journal of Heat and Mass Transfer80pp. 644-652 Elsevier

Simulations of a DC non-transferred arc plasma torch operating with argon–hydrogen have been performed by using a three-dimensional model. An artificially high electrical conductivity layer is employed to allow the current passing through the low temperature region near the anode wall. A new way by using two equations to describe the current density distribution is developed. Besides, a new method for determining the location of the arc-root attachment is proposed, in which the minimum total heat transfer rate through the anode wall is considered as the criterion for the lowest energy loss. Based on this criterion, the real arc core radius and length are predicted. Moreover, the influences of arc current and mass flow rate on the plasma arc characteristics are also investigated. The results obtained show that the location of the arc-root attachment predicted by the minimum total heat transfer rate principle is in good agreement with the previous work and the experimental data. Additionally, it is found that arc length can be reduced by increasing current or decreasing flow rate. Also, higher current and flow rate lead to higher temperature and velocity inside the plasma torch.

LM Armstrong, KH Luo, S Gu (2010)Two-dimensional and Three-dimensional Computational Studies of Hydrodynamics in the Transition from Bubbling to Circulating Fluidised Bed, In: Chemical Engineering Journal160(1, 15)pp. 239-248 Elsevier B.V.

This paper applies two-fluid modelling (TFM) to a two-dimensional and three-dimensional circulating fluidised bed (CFB). An energy minimisation multiscale (EMMS) based drag model is compared with a classical drag model, namely the Gidaspow model in the light of experimental data from the CFB. The axial particle velocities and the radial volume fraction at different heights are considered. The specularity coefficient responsible for the tangential solid velocities at the walls is varied to study the effect on the downflow of particles at the wall. The work is further extended to explore the effects of velocity variation on the flow distribution showing the transition from a bubbling to a fast fluidising regime. Furthermore, the diameters of the bubbles observed within the bubbling regime are compared with the Davidson’s bubble diameter model for a range of particle diameters. Varying the specularity coefficient showed that a free slip boundary condition underpredicted the downflow of particles at the wall and to add slight roughness to the wall gave a closer representation. The predictions for the 2D and 3D CFB axial velocities were in good agreement with the experimental data but the 2D results slightly overpredicted the core velocity. The transition from a bubbling to a fast fluidising regime as expected occurred once the inlet velocity exceeded the terminal velocity. The equivalent bubble diameter from the simulations agreed well with the calculated bubble diameter from the model.

B Liu, K Papadikis, Sai Gu, B Fidalgo, P Longhurst, Z Li, A Kolios (2017)CFD modelling of particle shrinkage in a fluidized bed for biomass fast pyrolysis with quadrature method of moment, In: Fuel Processing Technology164pp. 51-68 Elsevier

An Eulerian-Eulerian multi-phase CFD model was set up to simulate a lab-scale fluidized bed reactor for the fast pyrolysis of biomass. Biomass particles and the bed material (sand) were considered to be particulate phases and modelled using the kinetic theory of granular flow. A global, multi-stage chemical kinetic mechanism was integrated into the main framework of the CFD model and employed to account for the process of biomass devolatilization. A 3-parameter shrinkage model was used to describe the variation in particle size due to biomass decomposition. This particle shrinkage model was then used in combination with a quadrature method of moment (QMOM) to solve the particle population balance equation (PBE). The evolution of biomass particle size in the fluidized bed was obtained for several different patterns of particle shrinkage, which were represented by different values of shrinkage factors. In addition, pore formation inside the biomass particle was simulated for these shrinkage patterns, and thus, the density variation of biomass particles is taken into account.

S Gu, TJ Lu, AG Evans (2001)On the design of two-dimensional Cellular metals for combined heat dissipation and structural load capacity, In: International Journal of Heat and Mass Transfer44(11, Ju)pp. 2163-2175 ScienceDirect

Sandwich panels with two-dimensional metal cores can be used to carry structural load as well as dissipate heat through solid conduction and forced convection. This work attempts to uncover the nature of heat transfer in these lightweight systems, with emphasis on the effects of varying cell morphologies and cell arrangements. The types of cell shape and cell arrangement considered include regular hexagon, square with connectivity 4 or 3, and triangle with connectivity 6 or 4. Two analytical models are developed: corrugated wall and effective medium. The former models the cellular structure in detail whilst, the latter models the fluid saturated porous structure using volume averaging techniques. The overall heat transfer coefficient and pressure drop are obtained as functions of relative density, cell shape, cell arrangement, fluid properties, and overall dimensions of the heat sink. A two-stage optimization is subsequently carried out to identify cell morphologies that optimize the structural and heat transfer performance at specified pumping power and at lowest weight. In the first stage, the overall heat transfer performance is optimized against relative density. Regular hexagonal cells are found to provide the highest levels of heat dissipation. In the second stage, a constraint on stiffness is added. It is then found that, for panels with thin cores, triangular cells constitute the most compact and yet stiff heat sink design; however, for high heat flux scenarios, hexagonal cells outperform triangular and square cells.

F Xing, A Kumar, Y Huang, S Cahn, C Ruan, Sai Gu, X Fan (2017)Flameless Combustion with Liquid Fuel: A Review Focusing on Fundamentals and Gas Turbine Application, In: Applied Energy193pp. 28-51 Elsevier

Flameless combustion has been developed to reduce emissions whilst retaining thermal efficiencies in combustion systems. It is characterized with its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise and thermal stress for burners and less restriction on fuels (since no flame stability is required). Recent research has shown the potential of flameless combustion in the power generation industry such as gas turbines. In spite of its potential, this technology needs further research and development to improve its versatility in using liquid fuels as a source of energy. In this review, progress toward application of the flameless technique is presented with emphasis on gas turbines. A systematic analysis of the state-of-the-art and the major technical and physical challenges in operating gas turbines with liquid fuels in a flameless combustion mode is presented. Combustion characteristics of flameless combustion are explained along with a thorough review of modelling and simulation of the liquid fuel fed flameless combustion. A special focus is given to the relevant research on applications to the inner turbine burners. The paper is concluded by highlighting recent findings and pointing out several further research directions to improve the flameless combustion application in gas turbines, including in-depth flow and combustion mechanisms, advanced modelling, developed experimental technology and comprehensive design methods aiming at gas turbine flameless combustors.

Panneerselvam Ranganathan, Sai Gu (2017)Numerical Simulation of Catalytic Upgrading of Biomass Pyrolysis Vapours in a FCC Riser, In: Fuel Processing Technology171pp. 162-172 Elsevier

Catalytic upgrading of biomass pyrolysis vapours is a potential method for the production of hydrocarbon fuel intermediates. This work attempts to study the catalytic upgrading of pyrolysis vapours in a pilot scale FCC riser in terms of hydrodynamics, Residence Time Distribution (RTD) and chemical reactions by CFD simulation. NREL’s Davison Circulating Riser (DCR) reactor was used for this investigation. CFD simulation was performed using 2-D Eulerian–Eulerian method which is computationally less demanding than the alternative Euler-Lagrangian method. First, the hydrodynamic model of the riser reactor was validated with the experimental results. A single study of time-averaged solid volume fraction and pressure drop datas was used for the validation. The validated hydrodynamic model was extended to simulate hydrodynamic behaviors and catalyst RTD in the Davison Circulating Riser (DCR) reactor. Furthermore, the effects on catalyst RTD were investigated for optimising catalyst performance by varying gas and catalyst flow rates. Finally, the catalytic upgrading of pyrolysis vapours in the DCR riser was attempted for the first time by coupling CFD model with kinetics. A kinetic model for pyrolysis vapours upgrading using a lumping kinetic approach was implemented to quantify the yields of products. Five lumping components, including aromatic hydrocarbons, coke, non–condensable gas, aqueous fraction, and non–volatile heavy compounds (residue) were considered. It was found that the yield of lumping components obtained from the present kinetic model is very low. Thus, the further research needs to be carried out in the area of the kinetic model development to improve the yield prediction.

Vasileios Katranidis, Sai Gu, David Cox, Mark J Whiting, Spyros Kamnis (2018)FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings, In: Journal of Thermal Spray Technology27(5)pp. 898-908 Springer

The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W2C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape.

LM Armstrong, S Gu, KH Luo (2010)Study of wall-to-bed heat transfer in a bubbling fluidised bed using the kinetic theory of granular flow, In: International Journal of Heat and Mass Transfer53(21-22,)pp. 4949-4959 Elsevier B.V.

Research into heat transfer modelling in fluidised beds is very limited due to its complexity. The kinetic theory of granular flow (KTGF) has been applied successfully to hydrodynamic modelling in the past but its application in heat transfer modelling has not been tested extensively. A two-fluid Eulerian–Eulerian model has been carried out applying the KTGF to a wall-to-bed reactor. The local heat transfer coefficients are compared against experimental data for two drag models, namely the Gidaspow and the Syamlal–O’Brien drag models. Furthermore, a parametric study is carried out for a variety of coefficients of restitution, particle diameter sizes and inlet velocities. Near wall analysis is carried out in both dense and dilute regions. Both drag models detect the passage of the bubble reasonably well but they predict the complete transition of the bubble past the sensors occurs at slightly different times. The heat transfer coefficients obtained with the Syamlal–O’Brien model showed more local fluctuations than the Gidaspow model because the Syamlal–O’Brien models was developed based on the particle terminal velocities which would indicate a slight sensitivity to a microscopic scale. Extension of the simulation for a longer period makes it possible to reveal that a periodic distribution occurred after 1.5 s and the local heat transfer coefficients gradually reduced to agree better with the experimental results which were previously over estimated. The study shows that a regular dynamic pattern is established in the bubbling fluidised bed only after 1.5–2 s.

Daniel Sebastia Saez, Sai Gu (2017)CFD Modeling of Structured Packings at Small- and Meso-scale, In: Energy Procedia114pp. 1601-1614 Elsevier

This article presents a CFD model of the multiphase flow inside structured packings for amine-based post-combustion carbon capture. In the literature, simulations are performed at three scales due to computational limitations: small-, meso- and large-scale. This work focuses on small- and meso-scale, introducing interface tracking at both. The interfacial tracking is accomplished by using the Volume of Fluid (VoF) method. Small-scale allows studying the reaction kinetics of the absorption process in 2D geometries. Meso-scale has been used in the literature to describe the dry pressure drop of the packing (single-phase simulations). The interface tracking allows obtaining the relationship between the liquid load and both the liquid hold-up and the interface area. Data from the simulations are compared against experimental results found in the literature. The accurate modeling of the interface area, liquid hold-up and reaction kinetics allows utilizing this CFD model as a design tool for novel packings or to optimizing geometries already in use.

D. Sebastia Saez, S. Gu, P. Ranganathan (2014)Volume of Fluid Modeling of the Reactive Mass Transfer of CO2 Into Aqueous Amine Solutions in Structured Packed Elements at Micro-scale., In: Energy Procedia63pp. 1229-1242 Elsevier

This article presents a CFD model to describe the interfacial reactive mass transfer that takes place between a gas phase and a falling liquid film within a structured packing reactor. The simulations encompass the hydrodynamics, physical mass transfer and reaction kinetics. Regarding hydrodynamics, the liquid misdistribution phenomenon is represented and compared to experimental data found in the literature. Physical mass transfer is also implemented and an analysis of the influence of several parameters (e.g. amine concentration, gas pressure, gas velocity, flow configuration and contact angle) is carried out. Finally, the reactive mass transfer characteristics of the MEA-CO2 system are tested, showing the ability of the model to describe the values of the enhancement factor and the depletion of the solute in the bulk phase. The model is to be extended to meso-scale in the future to account for the performance of commercial structured packings.

L. Yang, L. Pastor-Pérez, S. Gu, A. Sepúlveda-Escribano, T. R. Reina (2018)Highly efficient Ni/CeO2-Al2O3 catalysts for CO2 upgrading via Reverse Water-Gas Shift: Effect of selected transition metal promoters, In: Applied Catalysis B: Environmental232pp. 464-471 Elsevier

In the context of Carbon Capture and Utilisation (CCU), the catalytic reduction of CO2 to CO via reverse water-gas shift (RWGS) reaction is a desirable route for CO2 valorisation. Herein, we have developed highly effective Ni-based catalysts for this reaction. Our study reveals that CeO2-Al2O3 is an excellent support for this process helping to achieve high degrees of CO2 conversions. Interestingly, FeOx and CrOx, which are well-known active components for the forward shift reaction, have opposite effects when used as promoters in the RWGS reaction. The use of iron remarkably boosts the activity, selectivity and stability of the Ni-based catalysts, while adding chromium results detrimental to the overall catalytic performance. In fact, the iron-doped material was tested under extreme conditions (in terms of space velocity) displaying fairly good activity/stability results. This indicates that this sort of catalysts could be potentially used to design compact RWGS reactors for flexible CO2 utilisation units.

LM Armstrong, S Gu, KH Luo (2010)The influence of multiple tubes on the tube-to-bed heat transfer in a fluidised bed, In: International Journal of Multiphase Flow36(11-12,)pp. 916-929 ScienceDirect

There have been few studies modelling both flow and heat transfer in fluidised beds. The kinetic theory of granular flow (KTGF) has been used for flow prediction in the past without heat transfer modelling. In the present study, a two-fluid Eulerian–Eulerian formulation incorporating the KTGF was applied first to a tube-to-bed reactor with one immersed tube and compared with the results in the literature. The bed was then modified to introduce two and three heated tubes. The effects on the flow and temperature distribution, local heat transfer coefficients and averaged heat transfer coefficients over a 3.0 s time period were carried out. Results showed that increasing the number of tubes promotes heat transfer from tubes to the particles and flow. The heat transfer coefficients extracted from the single-tube to three-tube cases were analysed in detail, confirming the importance of linking flow/particle and heat transfer calculations.

DK Shen, S Gu, B Jin, MX Fang (2010)Thermal degradation mechanisms of wood under inert and oxidative environments using DAEM methods, In: Bioresource Technology102(2, Jan)pp. 2047-2052 ScienceDirect

The pyrolytic behavior of wood is investigated under inert and oxidative conditions. The TGA experiment is given a temperature variation from 323 to 1173 K by setting the heating rate between 5 and 40 K/min. The results of DTG curves show that the hemicellulose shoulder peak for birch is more visible under inert atmosphere due to the higher content of reactive xylan-based hemicellulose (mannan-based for pine). When oxygen presents, thermal reactivity of biomass (especially the cellulose) is greatly enhanced due to the acceleration of mass loss in the first stage, and complex reactions occur simultaneously in the second stage when char and lignin oxidize. A new kinetic model is employed for biomass pyrolysis, namely the distributed activation energy model (DAEM). Under inert atmosphere, the distributed activation energy for the two species is found to be increased from 180 to 220 kJ/mol at the solid conversion of 10–85% with the high correlation coefficient. Under oxidative atmosphere, the distributed activation energy is about 175–235 kJ/mol at the solid conversion of 10–65% and 300–770 kJ/mol at the solid conversion of 70–95% with the low correlation coefficient (below 0.90). Comparatively, the activation energy obtained from established global kinetic model is correspondingly lower than that from DAEM under both inert and oxidative environments, giving relatively higher correlation coefficient (more than 0.96). The results imply that the DAEM is not suitable for oxidative pyrolysis of biomass (especially for the second mass loss stage in air), but it could represent the intrinsic mechanism of thermal decomposition of wood under nitrogen better than global kinetic model when it is applicable.

LM Armstrong, S Gu, KH Luo (2011)Parametric Study of Gasification Processes in a BFB Coal Gasifier, In: Industrial & Engineering Chemistry Research50(10)pp. 5959-5974 ACS Publications

An Eulerian-Eulerian computational fluid dynamics (CFD) model of the gasification processes in a coal bubbling fluidized bed (BFB) is presented based on the experimental setup taken from the literature. The base model is modified to account for different parameter changes in the model setup. The exiting gas compositions for the base model have been averaged over time and validated with experimental data and compared to the exiting results for the different parameter models. An extensive study is also carried out which considers the variation of different parameters such as bed temperatures, bed height, bed material, heat transfer coefficients, and devolatilization models influenced the gasification processes in different ways. Such an extensive parametric study has yet to be carried out for an Eulerian-Eulerian coal gasification model.

A Kumar, S Gu, H Tabbara, S Kamnis (2012)Study of Impingement of Hollow ZrO2 Droplets onto a Substrate, In: Surface and Coatings Technology220(15 Apr)pp. 164-169 ScienceDirect

In this paper a numerical study comparing the impingement behaviour of a hollow droplet and an analogous continuous droplet onto a substrate is presented. In the impingement model the transient flow dynamics during impact, spreading and solidification of the droplet are considered. The results of droplet spreading and solidification indicate that the impact process of the hollow droplet on the substrate is distinctly different from an analogous continuous droplet. The hollow droplet results in counter liquid jetting during the impact process, larger solidification time for the splat, smaller and thicker splat as compared to the analogous continuous droplet.

MH Duku, S Gu, EB Hagan (2010)A comprehensive review of biomass resources and biofuels potential in Ghana, In: Renewable and Sustainable Energy Reviews15(1, Jan)pp. 404-415 ScienceDirect

Biomass is the major energy source in Ghana contributing about 64% of Ghana's primary energy supply. In this paper, an assessment of biomass resources and biofuels production potential in Ghana is given. The broad areas of energy crops, agricultural crop residues, forest products residues, urban wastes and animal wastes are included. Animal wastes are limited to those produced by domesticated livestock. Agricultural residues included those generated from sugarcane, maize, rice, cocoa, oil palm, coconut, sorghum and millet processing. The urban category is subdivided into municipal solid waste, food waste, sewage sludge or bio-solids and waste grease. The availability of these types of biomass, together with a brief description of possible biomass conversion routes, sustainability measures, and current research and development activities in Ghana is given. It is concluded that a large availability of biomass in Ghana gives a great potential for biofuels production from these biomass resources.

H Tabbara, S Gu (2011)Modelling of Impingement Phenomena for Molten Metallic Droplets with Low to High Velocities, In: International Journal of Heat and Mass Transfer55(7-8 (M)pp. 2081-2086 ScienceDirect

Thermal spray coatings are formed by accelerating a stream of powder particles towards a targeted substrate surface where they impact, deform, and adhere. A fundamental understanding of the splat formation can pave the way for future developments in thermal spray technology through better understanding. Numerical modelling is applied in this investigation which simulates the detailed transient flow of a molten metal droplet impacting, deforming, and solidifying on a flat, solid substrate. The computations are carried out on a fixed Eularian structured mesh using a volume of fluid method to simulate the boundary between the metallic and atmospheric-gas phases. The results shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity.